Method for iso 26262-compliant evaluation of a pressure-sensor signal

ABSTRACT

A device and a method evaluate signals from one or more Wheatstone bridges. The requirements of ISO 26262 are taken into account by mixing a test signal with the measurement signal before amplification and before analog-to-digital conversion. After amplification and analog-to-digital conversion, the measurement signal and the test signal are unmixed again. If the test signal does not meet the expectation, the amplifier and/or the analog-to-digital converter is determined to be faulty.

CROSS REFERENCE TO RELATED APPLICATIONS

The present patent application is a national stage of, and claimspriority to, PCT Application No. PCT/DE2021/100681, filed on Aug. 10,2021, which application claims the priority of the German patentapplication 10 2020 123 930.0 filed Sep. 15, 2020, the disclosures ofwhich are incorporated by reference in the present patent application intheir entireties.

TECHNICAL FIELD

The disclosure discloses a method for monitoring a sensor system inoperation with a sensor element (WE), in particular a piezoresistiveWheatstone bridge (WB) of a pressure sensor, and the associated devicesand method modifications.

INTRODUCTION

Automotive applications typically employ numerous safety-relevantsensors, which the vehicle's control system must typically monitor forcorrect function during their operation. These include, for example,pressure-measuring devices in brake systems.

BACKGROUND

Various chopper methods for improving the signal-to-noise ratio inamplifiers are known from the prior art. Here, a multiplier upstream ofthe amplifier multiplies the signal by a chopper signal with a chopperfrequency, then amplifies it and then multiplies it again by the choppersignal. In the process, this multiplication mixes the amplified signalup on the one hand and also down on the other hand. Since only thedown-mixed portion is of interest, a low-pass filter suppresses thesignal components with the chopper frequency and all higher frequencies.This typically suppresses the 1/f noise of the amplifier stage.Combining such a process with a sensor element results in a low-noisesensor system.

Such a sensor system thus then implements a method for operating asensor system, in which the sensor system has a sensor element thatprovides an input signal with a time course of its input signal value.Such a sensor system comprises a signal path. The signal path includes,at a first location on the signal path, an amplifier having an input andan output. The signal path begins with the input signal. from the sensorelement and ends with a first output signal from the sensor system. Thevalue of the first output signal or the value of a signal derivedtherefrom represents the measured value. A first step of the prior anmethod is a first mixing of the signal in the signal path with thechopper signal at a second location in the signal path. Here, thissecond location in the signal path is between the input signal of thesensor element at the beginning of the signal path and the input of theamplifier located at the first location in the signal path. The choppersignal is typically mono-frequency. As a further step, the second mixingof the signal occurs at a third location in the signal path. The secondmixing of the signal is typically a mixing down of the signal with thechopper signal to a first demodulated signal. This third location is inthe signal path between the output of the amplifier, which is at thefirst location in the signal path, and the first output signal of thesensor system, which is at the end of the signal path. A first filteringof the first demodulated signal or a signal derived therefrom occurs ata fourth location in the signal path. The fourth location in the signalpath is between the third location in the signal path on the one handand the output signal of the sensor system at the end of the signal pathon the other hand. This first filtering is performed by applying a firstfilter function to the first demodulated signal or to a signal derivedtherefrom. The first filter function describes the relationship betweenthe time course of the first demodulated signal or the signal derivedtherefrom on the one hand and the time course of the signal immediatelyafter the first filtering on the other. The first output signal isdependent on this signal, which is the result of the first filtering, oris the result of this first filtering.

The first filter function F1[] is chosen so that essentially filteringthe chopper signal Cs with the first filter function F[] vanishes,F1[Cs]=0, and filtering a constant F1[1]=β₁ with β₁ as a real or complexvalue results in the form of a constant.

The disadvantage is that the control system typically must shut downsuch sensor systems for a functional check during operation.

SUMMARY

The task of the proposal is therefore to create a solution that does nothave this disadvantage of the prior art and has timber advantages.

To solve the problem, the paper presented herein proposes a method formonitoring a sensor system in operation, in which the sensor system hasa sensor element WB that provides an input signal Si with a time courseSi(t) of its input signal value. The sensor system has a signal path inwhich various device elements modify and evaluate the signal in thesignal path. At a first location in the signal path, the signal pathcomprises an amplifier DV having an input and an output. The signal pathbegins at the input signal Si, which is the output signal of the sensorelement WB. The signal path ends at the first output signal out1 of thesensor system. The value of the first output signal out1 of the sensorsystem or the value of a signal derived therefrom, if necessary, forexample by amplification, filtering or other further processing, thentypically represents the measured value. As an exemplary first step, theproposed method comprises a first mixing of the signal in the signalpath by means of a first mixer, for example a first multiplier M1, witha chopper signal Cs at a second location in the signal path, whichtypically differs from the first location at which the amplifier DV islocated. Preferably, therefore, the mixer or first multiplier M1 islocated at this second position in the signal path. Of course, it isalso conceivable to perform this mixing by an appropriate design of theamplifier DV in the amplifier DV, in which case, for example, the gainof the amplifier DV would depend on the chopper signal Cs. For example,for this purpose the amplifier DV may comprise a Gilbert multiplier asan amplifier stage. Instead, however, it has generally proved useful toimplement the signal path differentially and to realize the firstmultiplier Ml as a changeover switch which interchanges the two signalsof the differential signal in the signal path in dependence on thechopper signal Cs. This is particularly useful when a Wheatstone bridgeis used as sensor element WB, since this already provides a differentialsignal. This second location for performing this first mixing, forexample the position of the first multiplier, is typically located inthe signal path between the input signal Si, which is the output signalof the sensor element, at the beginning of the signal path and the inputof the amplifier DV at the first location in the signal path. For thesuppression of the 1/f noise to work reliably, the chopper signal Cs ispreferably band-limited or mono-frequency. This measure raises thesensor output signal, i.e., the time course Si(t) of the typically verylow frequency input signal Si, by the frequency of the chopper signal Csin the frequency spectrum. The subsequent amplifier stages,analog-to-digital converter stages and filter stages thus contaminatethe frequency range of the thus frequency-boosted input signal Si onlywith white noise and the signal-to-noise ratio improves. In order to beable to use the amplified and digitized sensor signal again, the sensorsystem must reverse this process. To do this, a second mixer, which istypically a second multiplier M2, typically performs a second mixing ofthe signal at a third location in the signal path with the choppersignal Cs to form a first demodulated signal DM1, Preferably, this thirdlocation in the signal path is between the output of the amplifier DV,which is indeed located at the first location in the signal path, andthe first output signal out1 of the sensor system at the end of thesignal path. A first filtering of the first demodulated signal DM1 or asignal derived therefrom takes place at a fourth position in the signalpath between the third position in the signal path on the one hand andthe output signal out1 of the sensor system at the end of the signalpath on the other hand. Typically, a first low-pass filter LP1 performsthis first filtering with a first filter function F1[]. Thus, this firstfiltering is performed by means of the application of said first filterfunction F1[] to the first demodulated signal DM1 or the signal derivedtherefrom by the first low-pass filter LP1. Instead of a low-passfilter, other filters are also conceivable depending on the application.However, the paper presented here assumes that the measured value to bedetermined changes cyclically only slowly and not predictably, and thattherefore the DC component of the measured value represents theessential information. The first filter function F1[] describes therelationship between the time course DM1(t) of the first demodulatedsignal DM1 or the signal derived from it on the one hand and the timecourse of the signal immediately after the first filtering on the otherhand. The first output signal out1 depends on this signal immediatelyafter the first filtering using the first filter function F1[]. However,the first output signal out1 can also be directly the result of thisfirst filtering using the first filter function F1[]. Now, in order tobe able to monitor the signal processing devices in the signal path toat least a large extent, in contrast to the prior art, the sensor systemfeeds a test signal TSS into the signal path. The sensor system takesthe modified test signal TSS from the signal path again after it haspassed through the signal path. The sensor system then evaluates thistest signal that has been taken out again. The design of the firstfilter function F1[] and the test signal TSS is preferably such that thefirst output signal out1 no longer contains any significant componentsof the test signal TSS A first low-pass filter LP1 implementing thefirst filter function F1[] in the sensor system thus blocks thetransmission of signal components corresponding to the test signal TSSfrom its input to its output. In order to perform this test signalinjection and extraction in the signal path, the proposed methodcomprises additional steps. In particular, this includes adding anorthogonal chopper signal Cs90 or a test signal TSS derived therefrom tothe signal in the signal path. This addition takes place at a fifthposition in the signal path. This fifth location is preferably betweenthe input signal Si, which is the output signal of the sensor elementWB, at the beginning of the signal path and the input of the amplifierDV at the first location in the signal path. The chopper signal Cs has atime course Cs(t) of the chopper signal Cs and the orthogonal choppersignal Cs90 has analogously a time course Cs90(t) of the orthogonalchopper signal Cs90. The time course Cs(t) of the chopper signal Cs mustsatisfy some conditions that this paper will state later. As long asthese conditions are satisfied, the choice of the time course of thechopper signal Cs is relatively free. However, the writing presentedhere recommends that the frequency bandwidth of the chopper signal notbe too wide, otherwise the response time of the sensor system maysuffer. The time course Cs90(t) of the orthogonal chopper signal Cs90must also satisfy some, albeit narrower, conditions, which this paperalso specifies later. As long as these conditions are satisfied, thechoice of the time course of the orthogonal chopper signal Cs90 isrelatively free. However, this paper recommends that the frequencybandwidth of the orthogonal chopper signal Cs90 should also not be toowide, otherwise the response time of the sensor system may also sufferunder certain circumstances. With respect to the said first filterfunction F1[], the time course Cs90(t) of the orthogonal chopper signalCs90 essentially has, except for noise and similar signal errors, theproperty F1[Cs90(t)×Cs(t)]=0 at least at times. This means that theorthogonal chopper signal Cs90 is orthogonal to the chopper signal Cs attypically predeterminable times.

For explanation we assume that X(t) is the time course of an arbitrary,not further defined signal. As an example, we assume that the firstfilter function F1[X] is the time indefinite integral of the time courseof the exemplary signal X(t). We thus assume that the following holds:

F1[X]=∫X dt

Under this condition, the following would then hold:

F1[Cs90(t)×Cs(t)]=∫Cs90(t)×Cs(t)dt

The first filter function F1[] in this example would then be nothingelse than the L2 product of the chopper signal Cs and the orthogonalchopper signal Cs90. For the L2 product, we refer for example tohttps://de.wikipedia.org/wiki/Lp-Raum#Der_Hilbertraum_L2 and there tothe section “The Hilbert space L²”. The L2 product is a scalar producton L². Another writing is for example the script “Introduction toDifferential Geometry” by Christopher R. Nerz, S198, definition X.1.5,which the reader can find at the time of this writing athttps://www.math.uni-tuebingen.de/de/forschung/gadr/lehre/sose2015/diffgeo.pdf.For example, it is conceivable that the chopper signal Cs follows a timesine function and the orthogonal chopper signal Cs90 follows a timecosine function. In such a case it is obvious that then the conditionF1[Cs90(t)×Cs(t)]=0 is not always satisfied, but only at certain times.If the said first low-pass filter LP1 executes the first filterfunction, it is thus useful if the design of the sensor system providesa holding circuit at the output of this first low-pass filter LP1. Theholding circuit samples the current value of the first filter functionF1[] of the first low-pass filter LP1 whenever the conditionF1[Cs90(t)×Cs(t)]=0 is satisfied. The holding circuit then freezes thiscurrent value at its output until the next time conditionF1[Cs90(t)×Cs(t)]=0 is satisfied. This sampling by the holding circuittransforms the indefinite integral of the example into a definiteintegral.

F1[Cs90(t)×Cs(t)]=∫₀ ^(T) ^(p) Cs90(t)×Cs(t)dt

Here it is assumed that the chopper signal Cs and the orthogonal choppersignal are periodic with respect to a common signal period T_(p). Thissampling of the filter output signal of a filter at times of theactually present orthogonality, i.e. the satisfaction of the boundaryconditions, shall also apply to the filters and their filteringmentioned in the following.

As a further step, a third mixing is performed The third mixing is amixing of the first demodulated signal DM1 or a signal derived therefromon the one hand with the orthogonal chopper signal Cs90 or with a signalderived from the orthogonal chopper signal Cs90 on the other hand. Thethird mixing generates a second demodulated signal DM2. In this thirdstep, a second filter function F2[] typically also filters the seconddemodulated signal DM2 or a signal derived therefrom to a second outputsignal out2 as a second filtering.

The second filter F2[] is typically chosen such that essentially theconditions F2[Cs(t)]=0 and F2[Cs90(t)]=0 and F2[Cs(t)×Cs90(t)]=0 andF2[1]=β₂ with β₂ as real or complex values hold. Furthermore, the firstfilter function F1[] is typically selected such that essentially theconditions F1[Cs(t)]=0 and F1[Cs90(t)]=0 and F1[Cs(t)×Cs90(t)]=0 andF1[1]=β₁ with β₁ as real or complex value hold. Here, preferably, asampling of the filter output signal of the second low-pass filter LP2with the second filter function F2[] always takes place at the times atwhich these conditions for the second filter function F2[] aresatisfied. Here, a sampling of the filter output signal of the firstlow-pass filter LP1 with the first filter function F1[] always takesplace analogously at the times at which these conditions for the firstfilter function F1[] are satisfied. The second output signal out2therefore preferably consists of the samples of the output value of thesecond filter function F2[DM2] of the second low-pass filter LP2, whichthe sensor system samples at times when the conditions for the secondfilter function F2[] are satisfied. The first output signal out1therefore preferably consists of the samples of the output value of thefirst filter function F1[DM1] of the first low-pass filter LP1, whichthe sensor system samples at times when the conditions for the firstfilter function F1[] are satisfied.

In order to conclude the correct function of the device parts in thesignal path, a first comparison of the value of the second output signalout2 or the value of a signal derived therefrom on the one hand with anexpected value interval on the other hand takes place. Furthermore, theconclusion on an error of a device part in the signal path is reached ifthe value of the second output signal out2 or the signal derivedtherefrom lies outside the expected value interval.

It is obvious to the person skilled in the art that, if necessary, hecan implement parts of the signal path in a signal processor and anassociated signal processor program. When talking, about a signal pathhere, the spatial positioning becomes a temporal positioning in case ofan implementation as a program in a signal processor. Thus, thepositions in the signal path then convert to processing times in thesequence of signal processing steps. Therefore, even if the subjectmatters to be protected here suggest a spatial positioning andarrangement from the wording, they also comprise a temporal positioningand sequence.

A Dicke method for white noise reduction can complement the proposedmethod, if necessary. The basic idea of a Dicke receiver is to comparethe DUT placed in a noisy environment with an equivalent noise source.

As a reference noise source, our example of a Wheatstone bridge WBtherefore uses a second Wheatstone bridge, the reference Wheatstonebridge RW, which the design of the sensor system preferably makescompletely the same and which the manufacturing process thereforetypically makes the same. The reference Wheatstone bridge RW cantypically but preferably does not show a measurement signal. Forexample, if the sensor element is a piezoresistive micromechanicalpressure sensor in which a Wheatstone bridge with piezoresistiveresistors is arranged on a diaphragm above a cavity, the referenceelement RW may be a second pressure sensor with a second Wheatstonebridge of exactly the same construction and preferably realized on thesame silicon crystal. In the now following proposal, the sensor systemwill generate a second output signal out2 indicating the differencebetween the output signal of the reference element, hereinafter referredto as reference signal Rs, and the output signal of the sensor element,herein the input signal Si. In case of equality of sensor element WB andreference element RW, this second output signal out2 should be zero.However, due to manufacturing tolerances and despite all closeness stillslightly different operating parameters like temperature and theunavoidable system noise, this second output signal out2 will never becompletely zero in reality. Rather, it will have to be within anexpected value interval in terms of value, which the sensor system cancheck. This is also true in the case where the reference element cannotprovide a measured value. In the case of the exemplary micromechanicalpressure sensor as sensor element, the reference element RW can alsocomprise, for example, only the reference Wheatstone bridge without adiaphragm and without a cavity, so that the influence of the pressure ismassively smaller. In this example, the reference Wheatstone bridge ismade equal (English “matching”) to the Wheatstone bridge. In thisexample of a piezoresistive pressure sensor, the Wheatstone bridge ofthe pressure sensor together with its diaphragm and its cavity, andtogether with the reference Wheatstone bridge, are accommodated on acommon silicon crystal. In this exemplary case, the reference Wheatstonebridge and the Wheatstone bridge then generate noise in the same way,which enables the elimination of noise.

The proposed noise reduction method therefore comprises, as a firststep, providing a reference element RW that provides a reference signalRs. This reference element RW may be, for example, the referenceWheatstone bridge mentioned by way of example. Analogous to theprocessing of the input signal Si in the signal path, a correspondingprocessing of the reference signal Rs takes place in a reference signalpath. It is of particular importance that the reference signal path isdesigned to be the same as the signal path for processing the inputsignal Si. This means that the reference signal path has positions ofprocessing of the reference signal in the reference signal path thatdirectly correspond to corresponding positions of processing of thesignal in the signal path. If a device in the reference signal pathperforms processing at a position in the reference signal path, acorresponding device in the signal path of the same design performs thesame processing of the signal in the signal path in the same manner.Thus, the signal processing of the reference signal Rs in the referencesignal path is initially a spatially parallel processing to the signalprocessing of the input signal Si in the signal path, which is performedin as much as possible the same way as the signal processing of theinput signal Si in the signal path.

An alternative design can now replace this space multiplex with a timemultiplex in certain parts of the signal path, if necessary, which hasthe advantage that the sensor system then uses not only identical deviceparts and process steps, but identical ones. Compared to the spacemultiplex, this increases the equality of the noise in the referencesignal path and in the signal path.

By space multiplex we understand here the temporally PARALLEL processingof signals in several identical or similar devices. In contrast, here weunderstand time-division multiplexing as the SERIAL processing ofsignals in one device. In the case of time-division multiplexing, theprocessing takes place in signal packets which are processed by the saiddevice one after the other in time.

The reference signal path starts at the reference element RW with thereference signal Rs. The reference signal path ends at the second outputsignal out2.

However, in order to use the reference element RW, the reference signalpath at the beginning of the reference signal path at the referencesignal Rs must be different from the signal path at the beginning of thesignal path at the input signal Si. In the proposal presented here, atleast the amplifier DV shall be common to the reference signal path andthe signal path. Thus, at a first location of the reference signal path,the reference signal path comprises the amplifier DV with the input andthe output. This first location of the reference signal path with theamplifier DV with its input and output is thus also the first locationof the signal path with the amplifier DV with its input and output.Thus, the amplifier DV is part of the reference signal path at a firstlocation in the reference signal path and at the same time also part ofthe signal path at the first location of the signal path. The referencesignal Rs is located at the beginning of the reference path. Thereference signal path has a sixth location in the reference signal pathbetween the reference signal Rs and the input of the amplifier DV at thefirst location in the reference path. The signal path has at acorresponding sixth location in the signal path between the input signalSi located at the beginning of the signal path and the input of theamplifier DV at the first location in the signal path, which is commonto the signal path and the reference signal path. In the referencesignal path, and in the signal path, at a common sixth location in thereference signal path and signal path, there is a switch DS common tothe signal path and the reference signal path and having a first inputand a second input. In the reference signal path, the common changeoverswitch DS is thus located at the sixth position in the reference signalpath. In the signal path, the common changeover switch DS is thuslocated at a corresponding sixth position in the signal path, which is acommon sixth position in the reference signal path and signal path.

The common changeover switch DS selects its active input depending on asecond chopper signal Cs2 between its first input and its second input.

The signal path comprises the first input of the changeover switch DS,while the reference signal path comprises the second input of thechangeover switch DS. Accordingly, the signal path does not include thesecond input of the changeover switch DS and the reference path does notinclude the first input of the changeover switch DS.

The common changeover switch DS selects its active input chosen independence on a second chopper signal Cs2 and accordingly switchesthrough the current value at this active input of the common changeoverswitch DS to its output of the common changeover switch DS.

The reference signal path and the signal path are thus identical in thesection from the output of the common changeover switch DS at the sixthposition in the reference signal path and signal path and the input ofthe amplifier DV at the first position in the reference signal path andsignal path.

However, the first filtering with the first filter function F1[] isexcluded and explicitly not part of the reference signal path.Typically, the exemplary first low-pass filter TP1 is not part of thereference signal path.

A fourth mixing of the first demodulated signal DM1 or a signal derivedtherefrom with the second chopper signal Cs2 generates a thirddemodulated signal DM3. This third mixing may take place in a thirdmixer, for example a third multiplier M3.

A third filtering of the third demodulated signal DM3 or of a signalderived therefrom by means of a third filter function F3[] to a thirdoutput signal out3 initially completes the signal processing here. Thisthird filtering can, for example, take place in a third low-pass filterLP3, which then implements the third filter function F3[].

The sensor system must ensure clean separation of i) the measured signalcomponent of the sensor element and ii) the differential signalcomponent from the difference between the measured signal component ofthe sensor element WB and the reference signal component of thereference element RW and iii) the test signal component. For thispurpose, a) the first filter function F1[] of the exemplary firstlow-pass filter LP1 and b) the second filter function F2[] of theexemplary second low-pass filter LP2 and c) the third filter functionF3[] of the exemplary third low-pass filter LP3 must satisfy certain,conditions.

Therefore, the design of the sensor system selects the first filterfunction F1[] such that essentially the following conditions aresatisfied:

F1[Cs(t)]=0 and F1[Cs2(t)]=0

F1[Cs90(t)]=0

F1[Cs(t)×Cs2(t)]=0

F1[Cs(t)×Cs90(t)]=0

F1[Cs2(t)×Cs90(t)]=0

F1[Cs(t)×Cs2(t)×Cs90(t)]=0

F1[1]=β₁

Here, β₁ is a real or complex value. As mentioned before, a device ofthe sensor system preferably samples the output of the exemplary firstlow-pass filter LP1 at exactly such times at which these conditions aresatisfied, if one disregards the unavoidable slight deviations due tonoise and manufacturing errors etc.

The design of the sensor System also selects the second filter functionF2[] such that essentially the following conditions are satisfied:

F2[Cs(t)]=0

F2[Cs2(t)]=0

F2[Cs90(t)]=0

F2[Cs(t)×Cs2(t)]=0

F2[Cs(t)×Cs90(t)]=0

F2[Cs2(t)×Cs90(t)]=0

F2[Cs(t)×Cs2(t)×Cs90(t)]=0

F2[1]=β₂.

Here, β₂ is a real or complex value, As mentioned before, a device ofthe sensor system preferably samples the output of the exemplary secondlow-pass filter LP2 at exactly those times when these conditions aresatisfied, in this regard, the technical teachings of this paperdisregard the unavoidable slight deviations due to noise andmanufacturing errors, etc. Similarly, the design of the sensor systemselects the third filter function F3[] such that substantially thefollowing conditions are satisfied:

F3[Cs(t)]=0

F3[Cs2(t)]=0

F3[Cs90(t)]=0

F3[Cs(t)×Cs2(t)]=0

F3[Cs(t)×Cs90(t)]=0

F3[Cs2(t)×Cs90(t)]=0

F3[Cs(t)×Cs2(t)×Cs90(t)]=0

F3[1]=β₃

Here, is a real or complex value. Preferably, a holding circuit of thesensor system samples the output of the exemplary third low-pass filterLP3 at sampling times. These sampling instants are thereby exactly suchinstants at which the above conditions are satisfied. In this context,the technical teachings of this document disregard the unavoidableslight deviations due to noise and manufacturing errors, etc. In orderto detect a defect of the reference element or the sensor element, asecond comparison of the value of the third output signal out3 or thevalue of a signal derived therefrom with a third expected value intervalis preferably performed. Furthermore, inference of an error occurs ifthe value of the third output signal out3 or the value of the signalderived therefrom is outside the third expected value interval. Forexample, a third comparator and a fourth comparator or a signalprocessor or the like may perform this comparison. The third comparatorthereby compares, if necessary, the value of the third output signalout3 with a third threshold value. If necessary, the fourth comparatorcompares the value of the third output signal out3 with a fourththreshold value.

The disadvantage of the above procedures is that the sensor element,here the exemplary Wheatstone bridge WB, is not part of the signal pathsection tested with the test signal TSS. The proposed proceduremodification now remedies this.

Thus, this paper now proposes a modified method for monitoring a sensorsystem in operation in which, as before, the sensor system comprises asensor element WB that provides an input signal Si having an inputsignal value in dependence on a test signal TSS.

As before, the sensor system has a signal path that again includes anamplifier DV having an input and an output at a first location in thesignal path.

As before, the signal path starts with the input signal Si from thesensor element WB and ends with a first output signal out1.

Here, too, the value of the output signal out1 represents the measuredvalue.

Again, a first mixing of the signal in the signal path with a choppersignal Cs takes place at a second position in the signal path. Thissecond position in the signal path is located between the input signalSi from the sensor element at the beginning of the signal path and theinput of the amplifier DV at the first position in the signal path.

The chopper signal Cs is again band-limited or mono-frequency. Again,the signal is mixed at a third position in the signal path with thechopper signal Cs to form a first demodulated signal DM1. The thirdposition in the signal path is located between the output of theamplifier DV at the first position in the signal path and the firstoutput signal out1 of the sensor system at the end of the signal path.

As before, a first filtering of the first demodulated signal DM1 or asignal derived therefrom is performed at a fourth position in the signalpath between the third position in the signal path on the one band andthe first output signal out1 at the end of the signal path on the otherhand. This first filtering is performed by applying a first filterfunction F1[] to the first demodulated signal DM1 or to a signal derivedtherefrom. The first filter function F1[] describes the relationshipbetween the time course DM1(t) of the first demodulated signal DM1 orthe signal derived therefrom on the one hand and the time course of thesignal on the other. This course of the signal is the one immediatelyafter the first filtering, i.e., typically the course of the filteroutput signal. The first output signal owl depends again on this signalimmediately after the first filtering or is itself the result of thisfirst filtering.

In contrast to the prior art, however, the test signal TSS is now alsogenerated in dependence on an orthogonal chopper signal Cs90. Thechopper signal Cs has a time course Cs(t) of the chopper signal Cs. Theorthogonal chopper signal Cs90 thereby has a time course Cs90(t) of theorthogonal chopper signal Cs90. The time course Cs90(t) of theorthogonal chopper signal Cs90 thereby has with respect to the saidfirst filter function F1[] essentially except for noise and similarsignal errors the property F1[Cs90(t)×Cs(t)]=0 at least at times. Atleast the time course Cs90(t) of the orthogonal chopper signal Cs90 hasthese properties at the times already discussed.

In contrast to the prior art, a third mixing of the first demodulatedsignal DM1 or a signal derived therefrom with the orthogonal choppersignal Cs90 or a signal derived therefrom and the generation of a seconddemodulated signal DM2 are further performed.

A second filtering of the second demodulated signal DM2 or a signalderived therefrom by means of a second. filter function F2[] results ina second output signal out2.

The second filter function F2[] is selected such that essentiallyF2[Cs(t)]=0 and F2[Cs90(t)]=0 and F2[Cs(t)×Cs90(t)]=0 and F2[1]=β₂ hold,with β₂ as real or complex value. As before, a second holding circuit(English: Sample & Hold) samples the output value of the exemplarysecond low-pass filter LP2, which preferably implements the secondfilter function F2[DM2], at times when these conditions are satisfied.

The first filter function F1[] is selected such that essentiallyF1[Cs(t)]=0 and F1[Cs90(t)]=0 and F1[Cs(t)×Cs90(t)]=0 F1[1]=β₁ hold,with β₁ as real or complex value. As before, a first holding circuitsamples the output value of the exemplary first low-pass filter LP1,which preferably implements the first filter function F1[DM1], at timeswhen these conditions are satisfied.

If necessary, a sensor device may include a first trigger circuit. Thefirst trigger circuit signals a sampling of the result of the firstfilter function F1[] to the first holding circuit at times when theconditions F1[Cs(t)]=0 and F1[Cs90(t)]=0 and F1[Cs(t)×Cs90(t)]=0F1[1]=β₁ are satisfied. Here, β₁ is a real or complex value, The firstholding circuit applies the sampling to the first demodulated signalDM1. The first holding circuit forms the first output signal out1 bythis sampling.

If necessary, a sensor device may comprise a second trigger circuit. Thesecond trigger circuit signals a sampling of the result of the secondfilter function F2[] to the second holding circuit at times when theconditions F2[Cs(t)]=0 and F2[Cs90(t)]=0 and F2[Cs(t)×Cs90(t)]=0 andF2[1]=β₂ are satisfied. Here, β₂ is a real or complex value. The secondholding circuit applies the sampling to the second demodulated signalDM2. The second holding circuit forms the second output signal out2 bythis sampling.

Finally, a first comparison of the value of the second output signalout2 or the value of a signal derived therefrom with an expected valueinterval is performed again and conclude an error if the value of thesecond output signal out2 or the signal derived therefrom is outside theexpected value interval. Here we refer to the explanations in thepreceding sections.

Again, it makes sense to include the sensor element and the referenceelement in the tested signal path. In this case, a reference element RWis also provided here, which supplies a reference signal Rs. The sensorsystem processes the reference signal Rs in the reference signal path.The reference signal path is again designed to be the same as the signalpath for processing the input signal Si. At this point, the writingpresented here refers to the method already described. Again thereference signal path starts with the reference signal Rs and ends againwith the second output signal out2.

As before, the reference signal path at the beginning of the referencesignal path at reference signal Rs and the signal path at the beginningof the signal path at input signal Si are different.

As before, the reference signal path at the first position in thereference signal path includes the amplifier DV with the input and theoutput. Thus, as above, the amplifier DV is also part of the referencesignal path at the first position in the reference signal path and atthe same time part of the signal path at the first position of thesignal path.

As before, the reference signal path has a sixth location in thereference signal path between reference signal Rs at the beginning ofthe reference signal path and input of amplifier DV at the firstlocation in the reference signal path, Again, the signal path has asixth location in the signal path between input signal Si at thebeginning of the signal path and input of the amplifier DV at the firstlocation in the signal path, which is common to the signal path and thereference signal path. Again, the reference signal path and the signalpath at this common sixth location in the reference signal path andsignal path comprise a changeover switch DS having a first input and asecond input, which is common to the signal path and the referencesignal path.

The changeover switch DS common to the signal path and the referencesignal path selects its active input among these two inputs, asdescribed above, depending on a second chopper signal Cs2 between itsfirst input and its second input.

The signal path again includes the first input of the changeover switchDS and does not include the second input of the changeover switch DS.The reference signal path accordingly includes the second input of thechangeover switch DS and does not include the first input of thechangeover switch DS.

The common changeover switch DS selects its active input chosen independence on a second chopper signal Cs2 and switches the value at thisactive input of the common changeover switch DS through to the output ofthe common changeover switch DS.

The reference signal path and the signal path are identical in thesection from the output of the common changeover switch DS at the sixthposition in the reference signal path and signal path and the input ofthe amplifier DV at the first position in the reference signal path andsignal path.

The first filtering with the first filter function F1[], for example thefirst low-pass filter LP1 is not part of the reference signal path.

A third filtering of the third demodulated signal DM3 or a signalderived therefrom by means of a third filter function F3[], for examplein a third low-pass filter LP3, generates a third output signal out3.

The sensor system must ensure clean separation of i) the measured signalcomponent of the sensor element and ii) the differential signalcomponent from the difference between the measured signal component ofthe sensor element WB and the reference signal component of thereference element RW and iii) the test signal component. For thispurpose, the first filter function F1[] of the exemplary first low-passfilter LP1 and the second filter function F2[] of the exemplary secondlow-pass filter LP2 and the third filter function of the exemplary thirdlow-pass filter LP3 must again satisfy certain conditions.

Therefore, the design of the sensor system selects the first filterfunction F1[] such that essentially the following conditions aresatisfied:

F1[Cs(t)]=0

F1[Cs2(t)]=0

F1[Cs90(t)]=0

F1[Cs(t)×Cs2(t)]=0

F1[Cs(t)×Cs90(t)]=0

F1[Cs2(t)×Cs90(t)]=0

F1[Cs(t)×Cs2(t)×Cs90(t)]=0

F1[1]=β₁

Here, β₁ is a real or complex value. As mentioned before, a samplingcircuit (English Sample&Hold) preferably samples the output of theexemplary first low-pass filter LP1 at exactly those times when theseconditions are satisfied. In doing so, the technical teachings of thispaper disregard the unavoidable slight deviations due to noise andmanufacturing errors, etc. Also, the design of the sensor system selectsthe second filter function F2[] such that substantially the followingconditions are satisfied:

F2[Cs(t)]=0

F2[Cs2(t)]=0

F2[Cs90(t)]=0

F2[Cs(t)×Cs2(t)]=0

F2[Cs(t)×Cs90(t)]=0

F2[Cs2(t)×Cs90(t)]=0

F2[Cs(t)×Cs2(t)×Cs90(t)]=0

F2[1]=β₂.

Here, β₂ is a real or complex value. As mentioned before, a samplingcircuit (English Sample&Hold) preferably samples the output of theexemplary second low-pass filter LP2 exactly those times when theseconditions are satisfied. The technical teachings of this paper therebydisregard the unavoidable slight deviations due to noise andmanufacturing errors, etc. in the same way, the design of the sensorsystem selects the third filter function F3[] such that essentially thefollowing conditions are satisfied:

F3[Cs(t)]=0

F3[Cs2(t)]=0

F3[Cs90(t)]=0

F3[Cs(t)×Cs2(t)]=0

F3[Cs(t)×Cs90(t)]=0

F3[Cs2(t)×Cs90(t)]=0

F3[Cs(t)×Cs2(t)×Cs90(t)]=0

F3[1]=β₃

Here, β₃ is a real or complex value. As mentioned before, a samplingcircuit (English Sample&Hold) preferably samples the output of theexemplary third low-pass filter LP3 at exactly those times when theseconditions are satisfied. The technical teachings of this paperdisregard the unavoidable slight deviations due to noise andmanufacturing errors etc. Now, in order to detect a defect of thereference element or the sensor element, a second comparison of thevalue of the third output signal out3 or the value of a signal derivedtherefrom with a third expected value interval preferably follows.Furthermore, inference of an error preferably follows if the value ofthe third output signal out3 or the value of the signal derivedtherefrom is outside the third expected value interval. For example, athird comparator may compare the value of the third output signal out3with a third threshold value, For example, a fourth comparator maycompare the value of the third output signal out3 with a fourththreshold value. For example, the third comparator and a fourthcomparator or a signal processor or the like may perform the secondcomparison.

In order to be able to carry out such procedures, special pressuresensors or sensors are advantageous.

Thus, a pressure sensor is proposed for use in a method according to oneor more of the previously described methods. The proposed pressuresensor comprises a Wheatstone bridge with four piezoresistive resistorsR1, R2, R3, R4 and a reference Wheatstone bridge with fourpiezoresistive reference resistors R5, R6, R7, R8. Preferably, thereference resistors R5, R6, R7, R8 of the reference Wheatstone bridge RWare arranged in the same way as the resistors R1, R2, R3, R4 of theWheatstone bridge WB. To achieve good thermal coupling and thus betternoise uniformity, the pressure sensor with the Wheatstone bridge WB asthe sensor element and the reference Wheatstone bridge RW as thereference element are arranged together on a monolithic crystal. Thismeans that they are exposed to approximately the same influences duringmanufacture and operation. The same orientation of the components andthe same arrangement of the components relative to each other maximizesthis equality.

The pressure sensor comprises at least one first cavity, which is dosedon at least one side by a first membrane and is surrounded by acontinuous wall. The cavity surface of the first cavity opposite thefirst diaphragm may be fully or partially open to allow access of amedium in the case of differential pressure sensors, or closed in thecase of absolute pressure sensors. The piezoresistive resistors R1, R2,R3, R4 of the Wheatstone bridge WB are preferably arranged at leastpartially on the first diaphragm. In this context, the documentpresented herein refers by way of example to the industrial propertyrights EP 2 524 389 B1, EP 2 524 390 B1, EP 2 524 198 B1, EP 2 523 896131 and EP 2 523 895 B1.

There are now several options for the reference sensor element:

A) The reference sensor element may be designed to provide a referencesignal Rs which should be equal to the input signal Si. In this case,the reference signal Rs and the input signal Si are equally dependent onthe value of the physical quantity that affects the respective outputsignal of the sensor element and the output signal of the referenceelement. Thus, a change in the value of this physical quantity thenresults in a change of equal value in the input signal Si and thereference signal Rs. In the example of a pressure sensor discussed here,this exemplary physical quantity is pressure,

B) The reference sensor element may be designed to provide a referencesignal Rs. This reference signal Rs should be different from the inputsignal Si in a predetermined manner, In this case, the input signal Siis provided by the sensor element, In this case, the reference signal Rsand the input signal Si are unequally dependent on the value of thephysical quantity which influences the respective output signal of thesensor element and the output signal of the reference element. Thus, achange in the value of this physical quantity results in a non-equalvalue change in the input signal Si and a non-vanishing change in thereference signal Rs. In the example of a pressure sensor discussed here,this exemplary physical quantity is typically pressure.

C) The reference sensor element may be designed to provide a referencesignal Rs which should be different from the input signal Si, namelysubstantially constant, in a manner known in advance. Thereby, thereference signal Rs is now preferably here substantially not dependenton the value of the physical quantity influencing the respective outputsignal of the sensor element, In contrast, the input signal Si continuesto depend on the value of the physical quantity affecting the respectiveoutput signal of the sensor element, so that a change in the value ofthis physical quantity results in a change in the input signal Si and noor only a negligible change in the reference signal Rs. In the exampleof a pressure sensor discussed here, this exemplary physical quantity ispressure,

Case A) Reference element and sensor element are of the same design.

In the pressure sensor example of case A, the pressure sensor comprisesa reference cavity closed on at least one side by a second membrane andsurrounded by a continuous wall. Preferably, the second membrane isconfigured the same as the first membrane. Preferably, the dimensionsand shape of the first membrane are equal to the dimensions and shape ofthe second membrane. Preferably, the reference cavity is configured inthe same manner as the first cavity. The cavity surface of the referencecavity opposite the second membrane may be fully or partially open toallow access to a medium, or closed. Preferably, this cavity surface ofthe reference cavity is closed when the corresponding cavity surface ofthe first cavity is closed. Preferably, this cavity surface of thereference cavity is open When the corresponding cavity surface of thefirst cavity is open, in which case the openings of the correspondingcavity surfaces are made in the same way. In the case of closedcavities, the first cavity and the reference cavity are preferablyfilled with the same gases at the same pressure or with preferably thesame vacuum. The piezoresistive reference resistors R5, R6, R7, R8 ofthe exemplary reference Wheatstone bridge RW are preferably arranged atleast partially on the second diaphragm above the reference cavity. Inthe optimum case, the behavior of the first Wheatstone bridge WB ininteraction with the first membrane and the first cavity coincide withthe behavior of the reference Wheatstone bridge RW in interaction withthe second membrane and the reference cavity, so that essentially nonon-zero signal other than noise can be measured at the third outputsignal out3. If a non-zero signal can be measured at the third outputout3 that lies outside the third expected value interval, an error ispresent,

Case B) Reference element and sensor element are designed unequally andthe reference element is sensitive to the physical quantity differentlythan the sensor element.

In this pressure sensor example of case B, the pressure sensor comprisesa reference cavity closed on at least one side by a second membrane andsurrounded by a continuous wall, in this case, the mechanical structureachieved by the reference cavity and the second membrane preferablydiffers from the mechanical structure achieved by the first cavity andthe first membrane. For example, the second membrane may be designeddifferently from the first membrane. For example, it may be thicker,thinner, larger, smaller, differently shaped or differently structured.The reference cavity may be shaped differently than the first cavity.For example, the reference cavity may be smaller or larger, deeper orshallower, or differently shaped or differently filled. The cavitysurface of the reference cavity facing the second membrane may be shapeddifferently than the cavity surface of the first cavity facing the firstmembrane. It may be closed while the other is open, or it may be openwhile the other is closed. If both are open, the shape, location of theopening within the respective cavity, and size may differ. In the caseof closed cavities, they may be filled with dissimilar gases and/or withdissimilar pressure, this writing understanding low pressure to includea vacuum. Of course, it is also conceivable that the piezoresistiveresistors R1, R2, R3, R4 of the Wheatstone bridge WB may be of adifferent design than the reference piezoresistive resistors R5, R6, R7,R8 of the reference Wheatstone bridge RW. This different design mayinvolve the resistor values, size, dimensions, designs, orientations,dopants, etc. In this case, the third output signal out3 and the firstoutput signal out1 together form an output signal vector whose outputsignal vector value may only be within predetermined ranges. It istherefore possible either to check the two-dimensional output signalvector value of this two-dimensional vector for agreement with atwo-dimensional expected value range, or to extract from thistwo-dimensional output signal vector value the two values of twodifferent physical parameters influencing the sensor element and thereference element differently. If the two-dimensional value of thetwo-dimensional output vector leaves the two-dimensional expected valuerange, the sensor system or a higher-level computer system can concludethat an error has occurred.

Case C) Reference element and sensor element are not equal and thereference element is not sensitive to the physical quantity.

In case C, the pressure sensor preferably does not comprise a referencecavity. As a result, the mechanical structure of the reference elementin the form of the reference Wheatstone bridge RW deviates massivelyfrom the mechanical structure of the sensor element in the form of theWheatstone bridge WB. Case C is ideally an extreme case of case B. Inthis extreme case, the reference element then typically no longer hasany sensitivity to the physical quantity. In this case, the referenceelement has the form of the reference Wheatstone bridge RW. The physicalquantity here is the physical quantity that the sensor system is todetect by means of the sensor element, in this case in the form of theWheatstone bridge WB. The reference sensor element RW then typicallydetects parasitic parameters, such as pressure or humidity. Theevaluation is analogous to the evaluation in case B.

Preferably, in this exemplary case of a micromeehanical pressure sensor,the piezoresistive reference resistors R5, R6, R7 RS are arranged suchthat a deflection of the first diaphragm does not affect the referenceresistors R5, R6, R7, R8 of the reference Wheatstone bridge RW.Preferably, the reference resistors R5, R6, R7, R8 are not located onthe first diaphragm for this purpose.

The first resistor R1 of the Wheatstone bridge WB resembles a fifthresistor R5 of the reference Wheatstone bridge WB in that it isconstructed in the same manner.

The second resistor R2 of the Wheatstone bridge WB resembles a sixthresistor R6 of the reference Wheatstone bridge WB in that it isconstructed in the same manner.

The third resistor R3 of the Wheatstone bridge WB resembles a seventhresistor R7 of the reference Wheatstone bridge WB in that it isconstructed in the same manner.

The fourth resistor R4 of the Wheatstone bridge WB resembles an eighthresistor R8 of the reference Wheatstone bridge WB in that it isconstructed in the same manner.

The sensor system then preferably uses this reference element in thisexample in the form of a reference Wheatstone bridge RW as a referencenoise source for the subsequent signal processing of the input signal Sifrom the sensor element, in this case the Wheatstone bridge WB.

This paper thus proposes a sensor for use in one of the methodspreviously presented. In particular, the sensor may be a pressuresensor. The proposed sensor comprises a first resistor R1 having a firstterminal and a second terminal. The proposed sensor comprises a secondresistor R2 having a first terminal and a second terminal. The proposedsensor has a third resistor R3 having a first terminal and a secondterminal. The proposed sensor has a fourth resistor R4 having a firstterminal and a second terminal. Now, in order to be able to generate afirst differential modulation voltage V_(mod1), the sensor in thisexample comprises a first voltage source V1 having a first terminal anda second terminal and a second voltage source V2 having a first terminaland a second terminal. The first terminal of the first voltage source V1is connected to a first supply voltage line VDD. The second terminal ofthe first voltage source V1 is connected to the first terminal of thefirst resistor R1. The second terminal of the first resistor R1 isconnected to the first terminal of the second resistor R2. The secondterminal of the second resistor R2 is connected to a second supplyvoltage line GND. The first terminal of the second voltage source V2 isconnected to the first supply voltage line VDD. The second terminal ofthe second voltage source V2 is connected to the first terminal of thethird resistor R3. The second terminal of the third resistor R3 isconnected to the first terminal of the fourth resistor R4. The secondterminal of the fourth resistor R4 is connected to the second supplyvoltage line GND. The first voltage of the first voltage source V1depends on the test signal TSS. The second voltage of the second voltagesource V2 depends on the test signal TSS in the opposite way as thefirst voltage of the first voltage source V1.

Instead of feeding a test signal via voltage sources V1, V2, V1 b, V2 b,the signal can also be fed via corresponding current source pairs, inwhich case the sensor system must excite the Wheatstone bridge WB andthe reference Wheatstone bride RW not from voltage source pairs [V1,V2], [V1 b, V2 b], but from current source pairs. The Wheatstone bridgeWS is thereby assigned a first current source pair. The referenceWheatstone bridge RW is assigned a second current source pair. Eachcurrent source pair of these current source pairs then consists of twocurrent sources each. From the current sources of such a current sourcepair, a first current source of this current source pair feeds a firstcurrent into a first branch of the Wheatstone bridge WB or the referenceWheatstone bridge RW, depending on the assignment. Of the currentsources of this current source pair, the second current source of thiscurrent source pair feeds a second current into the second branch of theWheatstone bridge WB or the reference Wheatstone bridge RW, depending onthe assignment. The first current and the second current depend on thetest signal TSS with different signs. In total, the sensor system thenrequires four current sources, which the design of the sensor systempreferably makes equal again (English: matching), For this purpose, thisdocument does not include a drawing, because this possibility is obviousto the person skilled in the art.

Instead of feeding the test signal component into the input signal Si byvoltage or current sources, it is also possible to modulate the value ofthe resistors R1, R2, R3, R4 of the Wheatstone bridge WB and thereference resistors R5, R6, R7, R8 of the reference Wheatstone bridgeRW. To this end, the document presented herein proposes here, as afurther example, a sensor, in particular a pressure sensor, typicallyintended for use in a method according to one or more of the previouslypresented methods. The sensor then comprises a first resistor R1 havinga first terminal and a second terminal, and a second resistor R2 havinga first terminal and a second terminal, and a third resistor R3 having afirst terminal and a second terminal, and a fourth resistor R4 having afirst terminal and a second terminal. Further, the sensor comprises afirst variable resistor RV1 having a first terminal and a secondterminal and a second variable resistor RV2 having a first terminal anda second terminal.

The first terminal of the first variable resistor RV1 is connected tothe first supply voltage line VDD. The second terminal of the firstvariable resistor RV1 is connected to the first terminal of the firstresistor R1. The second terminal of the first resistor R1 is connectedto the first terminal of the second resistor R2. The second terminal ofthe second resistor R2 is connected to a second supply voltage line GND.The first terminal of the second variable resistor RV2 is connected tothe first supply voltage line VDD. The second terminal of the secondvariable resistor RV2 is connected to the first terminal of the thirdresistor R3. The second terminal of the third resistor R3 is connectedto the first terminal of the fourth resistor R4. The second terminal ofthe fourth resistor R4 is connected to the second supply voltage lineGND. The resistance value of the first variable resistor RV1 depends ona test signal TSS, and the resistance value of the second variableresistor RV2 depends on the test signal TSS in the opposite way to theresistance value of the first variable resistor RV1. Preferably, thedesign of the sensor system implements the first variable resistor RV1and the second variable resistor RV2 identically (English: matching).

Such methods and the exemplary devices of the following figures, atleast in some implementations, enable verification of the signal pathduring operation. However, the advantages are not limited to this.

BRIEF DESCRIPTION OF THE DRAWINGS

The figures represent exemplary designs of the proposals of this paper.They are schematic and simplified.

FIG. 1 shows a simple, exemplary example of the disclosure.

FIG. 2 shows exemplary waveforms for the operation of a device accordingto FIG. 1 .

FIG. 3 shows a view corresponding to FIG. 1 , wherein the first adderand the first multiplier have swapped order,

FIG. 4 shows essentially the exemplary signals of FIG. 2 but now adaptedto FIG.

FIG. 5 shows a view based on FIG. 3 .

FIG. 6 shows a view largely corresponding to FIG. 5 with the differencethat the excitation voltages of the Wheatstone bridge and the referenceWheatstone bridge are modulated in FIG. 6 .

FIG. 7 shows a view largely corresponding to FIG. 6 with the differencethat the resistors of the Wheatstone bridge and the reference Wheatstonebridge are modulated in FIG. 7 .

DESCRIPTION FIG. 1

FIG. 1 shows a simple, exemplary example of the disclosure. A Wheatstonebridge WB is selected as an exemplary sensor having a differentialoutput. The exemplary Wheatstone bridge WB comprises a first resistorR1, a second resistor R2, a third resistor R3 and a fourth resistor R4.For example, when used in a piezoresistive pressure sensor, the firstresistor R1 is a piezoresistive first resistor R1 and the secondresistor R2 is a piezoresistive second resistor R2 and the thirdresistor R3 is a piezoresistive third resistor R3 and the fourthresistor R4 is a piezoresistive fourth resistor R4. The first resistorR1 is connected in series with the second resistor R2 between the firstsupply voltage line VDD and the second supply voltage line GND. Thethird resistor R3 is also connected in series with the fourth resistorR4 between the first supply voltage line VDD and the second supplyvoltage line GND. As an example, the sensor system operates theWheatstone bridge WB with the supply voltage between the first supplyvoltage line VDD and the second supply voltage line GND. Accordingly,the Wheatstone bridge WB has a first terminal connected to the supplyvoltage line VDD and a second terminal connected to the second supplyvoltage line GND. The node between the first resistor R1 and the secondresistor R2 forms a negative input signal Sin of the differential inputsignal Si, as an example. The node between the third resistor R3 and thefourth resistor R4 forms a positive input signal Sip of the differentialinput signal Si, by way of example.

The use of Wheatstone bridges is known from many sensor systems forconverting the physical parameter in question into a differentialvoltage signal between a positive input signal Sip and a negative inputsignal Sin, For example, the Wheatstone bridge WB may be such a bridgeof piezoresistive resistors R1, R2, R3, R4 of the sensor elements of apiezoresistive micromechanical pressure sensor or the like.

The signal pair of positive input signal Sip and negative input signalSin forms the differential input signal Si. In the example of Figure afirst adder Al adds a differential test signal TSS to the differentialinput signal Si to form the differential input signal with test signalcomponent SiT.

The first multiplier M1 multiplies the differential input signal withtest signal component SiT by a chopper signal Cs and thus forms thedifferential, multiplied input signal with test signal component MSiT.Preferably, the chopper signal Cs is a digital signal with two logicalvalues, here exemplarily assigned with 0 and 1. For example, a switchingdevice can firm the first multiplier M1. The design then forms thefunction of the switching device as follows, for example:

A) With a logical 0 as the value of the chopper signal Cs, thedifferential multiplied input signal with test signal component MSiTcorresponds to the differential input signal with test signal componentSiT,

B) With a logical 1 as the value of the chopper signal Cs, thedifferential multiplied input signal with test signal component MSiTcorresponds to the differential input signal with test signal componentSiT with swapped lines.

A differential amplifier DV amplifies the differential, multiplied inputsignal with test signal component MSiT to an amplifier output signal VO.

An analog-to-digital converter ADC converts the amplifier output signalVO into an input signal DFI of a digital filter DF. This input signalDFI of the digital filter DF is typically a digital signal of samples ofthe amplifier output signal VO from the analog-to-digital, converterADC.

The digital filter DF filters the input signal DFI of the digital filterDF to an output signal DFO of the digital filter DF. In doing so, thedigital filter suppresses any signal components that may be atinterference frequencies. Typically, this is a decimation filter for theconversion artifacts added by the sampling by means of theanalog-to-digital converter ADC.

An exemplary phase compensator PC corrects the resulting phase errorsand forms a phase compensator output signal PCO.

After amplification and digitization, a second multiplier M2 multi pliesthe phase compensator output signal PCO with the chopper signal CS toform a first demodulated signal DM1.

A first low-pass filter LP1 suppresses in the first demodulated signalDM1 the frequencies corresponding to the frequencies in the signalspectrum of the chopper signal Cs. The first low-pass filter LP1 alsosuppresses the frequencies corresponding to the frequencies in thesignal spectrum of an orthogonal chopper signal Cs90. Furthermore, thefirst low-pass filter LP1 suppresses the mixed frequencies that mayresult from a multiplication of the chopper signal Cs by the orthogonalchopper signal Cs90. The first low-pass filter LP1 suppresses thesesignal components except for a DC component in the first demodulatedsignal DM1. The first low-pass filter LP1 thus forms a first outputsignal out1 whose value corresponds to the value of the differentialinput signal Si.

A third multiplier M3 mixes the first demodulated signal DM1 with theorthogonal chopper signal Cs90 to form a second demodulated signal DM2.A second low-pass filter LP2 suppresses in the second demodulated signalDM2 the frequencies corresponding to the frequencies in the signalspectrum of the chopper signal Cs. The second low-pass filter LP2further suppresses the frequencies corresponding to the frequencies inthe signal spectrum of the orthogonal chopper signal Cs90. The secondlow-pass filter LP2 also suppresses the mixed frequencies that may beproduced by multiplying the chopper signal Cs by the orthogonal choppersignal Cs90. The second low-pass filter LP2 suppresses these frequenciesexcept for a DC component in the second demodulated signal DM2. Thesecond low-pass filter LP2 thus forms a second output signal out2.

In the example of FIG. 1 , a signal generator G1 generates the choppersignal Cs and the orthogonal chopper signal Cs90. Preferably, thechopper signal Cs is bandwidth-limited or monofrequency. Preferably, theorthogonal chopper Cs90 is also bandwidth-limited or monofrequency.Preferably, the orthogonal chopper signal Cs90 is different from thechopper signal Cs. The first low-pass filter LP1 has a first filterproperty in the form of a first filter function F1[] such that:out1=F1[DM1]. The second low-pass filter LP2 has a filter property inthe form of a second filter function F2[] such that: out2=F2[DM2],Typically, the first low-pass filter LP1 and the second low-pass filterLP2 have a very preferably equal filter property in the form of an equalfilter function F[]=F1[]=F2[].

The chopper signal CS and the orthogonal chopper signal Cs90 shall beorthogonal to each other with respect to the first filter LP1 and withrespect to the second filter LP2. That is, The following should hold atleast at preferred times:

a) F1[Cs(t)×Cs90(t)]=0

b) F2[Cs(t)×Cs90(t)]=0

Here Cs(t) shall represent the time course of the values of the choppersignal Cs and Cs90(t) shall represent the time course of the values ofthe orthogonal chopper signal Cs90.

Furthermore, the first filter function F1[] shall preferably be anessentially linear filter function. That is, the following should holdfor a. signal sum of any first example signal X1(t) and any secondexample signal X2(t) and fur a real number α:

A) F1[X1(t)+X2(t)]=B1[X1(t)]+F1[X2(t)]

B) F1[α×X1(t)]=α×F1[X1]

Furthermore, the second filter function F2[] shall preferably be anessentially linear filter function. The following should hold for asignal sum of any first example signal X1(t) and any second examplesignal X2(t) and for a real number α:

C) F2[X1(t)+X2(t)]=B2[X1(t)]+F2[X2(t)]

D) F2[α×X1(t)]=α×F2[X1]

Finally, the first filter function F1[] and the second filter functionF2[] shall each have a low-pass property. That is, the following shouldhold:

F1[1]=β₁ and F2[1]=β₂ with β₁ as a real non-zero constant and β₂ as areal non-zero constant.

For example, the chopper signal Cs can be a mono-frequency PWM signalwith the values −1 and 1 and a duty cycle of 50% and a chopper signalfrequency. The orthogonal chopper signal Cs90 can then be, for example,a +/−90° phase-shifted signal with the values −1 and 1 and a duty cycleof 50%. Alternatively, it can be a mono-frequency PWM signal with thevalues −1 and 1 and a duty cycle of 50% and a signal frequency that is,for example, an integer multiple of the chopper signal frequency. Thechopper signal Cs can also be a band-hunted, non-mono-frequency signal.The only important thing is that the orthogonality conditions aresatisfied. Typically, the chopper signal Cs is periodic and theorthogonal chopper signal Cs90 is periodic. If necessary, it is usefulto provide the first low-pass filter LP1 and the second low-pass filterLP2 each with a holding circuit (English: sample & hold) at theirrespective outputs. In this case, it is useful to sample the output ofthe first low-pass filter LP1 with a first holding circuit of theholding circuits at times when the orthogonality conditions a) and b)are satisfied. The first holding circuit outputs the first value sampledin this way as the value of the first output signal out1 until the nexttime orthogonality conditions a) and b) are satisfied. In the said case,it is also useful to sample the output of the second low-pass filter LP2with a second holding circuit of these holding circuits at times whenthe orthogonality conditions a) and b) are satisfied. The second holdingcircuit then outputs the second value sampled in this way as the valueof the second output signal out2 until the next time that theorthogonality conditions a) and b) are satisfied.

In the example of FIG. 1 , a test signal generator TSG, which can alsobe part of the signal generator G1, generates the test signal TSS fromthe orthogonal chopper signal Cs90. The test signal generator TSGtypically sets the amplitude according to a preset.

The design may also implement the digital filter DF, the phasecompensation PC, the signal generator G1, the test signal generator TSG,the second multiplier M2, the third multiplier M3, the first low-passfilter LP1 and the second low-pass filter LP2, for example, by means ofa digital circuit or by means of a signal processor system withappropriate programming,

Preferably, a comparison means, for example a first comparator incooperation with a second comparator or said signal processor, comparesthe value of the second output signal with an expected value rangebounded by a first expected value and a second expected value. If thevalue of the second output signal is between the first expected valueand the second expected value, i.e., within the expected value range,the input stage comprising the first multiplier M1, the differentialamplifier DVI, the analog-to-digital converter ADC, the digital filterDF, the phase compensator PC and the second multiplier M2 is presumablyoperating correctly. Thus, the sensor system can make a statement aboutthe presumably correct function of the input stage in this way.

FIG. 2

FIG. 2 shows exemplary waveforms for the operation of a device accordingto FIG. 1 . The levels are chosen arbitrarily. The line dashed withshorter bars is intended to represent the respective zero line. The linedashed with longer dashes shall represent the respective line of themean value.

FIG. 3

FIG. 3 largely corresponds to FIG. 1 , but now the first adder A1 andthe first multiplier M1 have swapped order in the signal path from thesensor to the first output signal out1. This has the advantage that thedesign of the sensor system can easily integrate the first adder A1 intothe input stage of the differential amplifier DV. However, this has thedisadvantage that the test signal TSS now no longer co-tests the firstmultiplier M1. In addition, a further multiplier is required within thetest signal generator TSG, which multiplies the orthogonal choppersignal Cs90 with the chopper signal, (Cs) and processes it to the testsignal TSS.

FIG. 4

FIG. 4 shows essentially the exemplary signals of FIG. 2 but now adaptedto FIG. 3 .

FIG. 5

FIG. 5 is based on FIG. 3 . In the example of FIG. 5 , a referenceWheatstone bridge RW is also provided.

A Wheatstone bridge WB is again selected as an exemplary sensor with adifferential output. The Wheatstone bridge WB comprises a firstpiezoresistive resistor R1, a second piezoresistive resistor R2, a thirdpiezoresistive resistor R3 and a fourth piezoresistive resistor R4. Thefirst resistor R1 is connected in series with the second resistor R2between the first supply voltage line VDD and the second supply voltageline GND. The third resistor R3 is also connected in series with thefourth resistor R4 between the first supply voltage line VDD and thesecond supply voltage line GND. The sensor system operates theWheatstone bridge WB with the supply voltage as an example. Accordingly,the Wheatstone bridge WB has a first terminal connected to the firstsupply voltage line VDD and a second terminal connected to the secondsupply voltage line GND. The node between the first resistor R1 and thesecond resistor R2 forms the negative input signal Sin of thedifferential input signal Si, as an example. The node between the thirdresistor R3 and the fourth resistor R4 forms the positive input signalSip of the differential input signal Si, by way of example.

Many sensor systems use Wheatstone bridges to convert the relevantphysical parameter into a differential voltage signal between a positiveinput signal Sip and a negative input signal Sin. For example, theWheatstone bridge WB can be such a bridge of piezoresistive resistors ofthe sensor elements of a piezoresistive micromechanical pressure sensoror the like. At this point, reference is made, for example, to theindustrial property rights EP 2 524 389 B1.

EP 2 524 390 B1, EP 2 524 198 B1, EP 2 523 896 B1 and EP 2 523 895 B1 asexamples of such pressure sensors.

The reference Wheatstone bridge RW comprises, for example, a fifthpiezoresistive resistor R5, a sixth piezoresistive resistor R6, aseventh piezoresistive resistor R7 and an eighth piezoresistive resistorR8. The fifth, resistor R5 is connected in series with the sixthresistor R6 between the first supply voltage line VDD and the secondsupply voltage line GND. The seventh resistor R7 is also connected inseries with the eighth resistor R8 between the first supply voltage lineVDD and the second supply voltage line GND. The sensor system operatesthe reference Wheatstone bridge RW with the supply voltage as anexample. Accordingly, the reference Wheatstone bridge RW is connectedwith a first terminal to the first supply voltage line VDD. Thereference Wheatstone bridge RW has a second terminal connected to thesecond supply voltage line GND. The node between the fifth resistor R5and the sixth resistor R6 exemplarily forms a negative reference signalRin of a differential reference signal Rs. The node between the seventhresistor R7 and the eighth resistor 8 exemplarily forms a positivereference signal Rip of the differential reference signal Rs. The fifthresistor R5 and the sixth resistor R6 and the seventh resistor R7 andthe eighth resistor R8 represent the resistors of the referenceWheatstone bridge RW. The first resistor R1 and the second resistor R2and the third resistor R3 and the fourth resistor R4 represent theresistors of the Wheatstone bridge WB. Preferably, the manufacturingprocess manufactures the resistors of the reference Wheatstone bridge RWusing the same steps simultaneously with the resistors of the Wheatstonebridge WB. For example, the Wheatstone bridge WB may be the sensingbridge of a piezoelectric micromechanical pressure sensor, for exampleon a silicon single crystal piece. In such an example, the manufacturingprocess according to the design preferably manufactures the referenceWheatstone bridge RW on the same silicon single crystal piece togetherwith the Wheatstone bridge WB. In such an example, the manufacturingprocess according to the design preferably also manufactures in the sameorientation of the corresponding resistors. The technical term for thisis “matching”. For example, the reference Wheatstone bridge RW may bepart of a second pressure sensor similar to the pressure sensor of theWheatstone bridge WB and fabricated in the same substrate, for examplein the same semiconductor crystal. In that case, a third output signal,out3, that represents the value of the difference between thedifferential value of the reference signal Rs and the differential valueof the differential input signal Si should be close to zero.

In the case of a reference Wheatstone bridge WB equal to the Wheatstonebridge, a deviation between the differential value of the referencesignal Rs and the differential value of the differential input signal Sirepresented by the value of the third output signal out3 should thendisappear. A value of the third output signal out3 outside a permissibleexpected value range around this zero value then indicates an error inthe case of a reference Wheatstone bridge WB designed equal to theWheatstone bridge. In this case, by the way, the value of the firstoutput signal indicates only the average between the signal componentbased on the value of the input signal Si and the signal component basedon the value of the reference signal. In this respect the system of FIG.5 has the disadvantage that the sensitivity is typically halved. Onlywhen the reference Wheatstone bridge RW is subjected to the influence ofthe influencing physical parameter in the same way as the Wheatstonebridge WB, the reference Wheatstone bridge RW changes its referencesignal RS in. dependence on the value of this physical parameter in thesame way as the Wheatstone bridge WE changes the input signal Si independence on this physical parameter. Only then does the value of thefirst output signal out1 at maximum sensitivity corresponds to the valueof this physical parameter. The physical parameter can be, for example.a pressure in the case of pressure sensors.

However, in the case of a reference Wheatstone bridge RW that is notequal to the Wheatstone bridge WB, a deviation between the differentialvalue of the reference signal Rs and the differential value of thedifferential input signal Si represented by the value of the thirdoutput signal out3 can then no longer disappear. If the referenceWheatstone bridge is substantially insensitive to the physical parameterto be sensed by the Wheatstone bridge WB, the value of the third outputsignal out3 typically reflects a value for that physical parameter thatis typically substantially adjusted for such influencing, factors thataffect the reference Wheatstone bridge RW and the Wheatstone bridge inthe same manner.

However, it is also conceivable that in the example of a micromechanicalpressure sensor, the reference Wheatstone bridge RW is not located on amembrane. In such a case, the reference Wheatstone bridge RW should thusshow essentially no pressure-dependent signal. The measured value thenappears in the example of FIG. 5 as the value of the third output signalout3.

The advantage of the arrangement of FIG. 5 is that with a good thermalcoupling of the reference Wheatstone bridge RW with the Wheatstonebridge WB, the reference Wheatstone bridge RW noises in the same way asthe Wheatstone bridge WB itself. This allows the sensor system to alsoreliably suppress the 1/f noise of the Wheatstone bridge WB itself,which is not possible in FIG. 1 .

The signal pair of positive input signal Sip and negative input signalSin forms the differential input signal Si. The signal pair of positivereference signal Rip and negative reference signal Rin forms thedifferential reference signal Rs.

A Dicke switch DS switches between the differential input signal Si andthe differential reference signal Rs dependent the second chopper signalCs2.

In the example of FIG. 5 , a first multiplier M1 multiplies theresulting mixed signal of differential input signal Si and differentialreference signal Rs by the chopper signal Cs to form the multipliedinput signal MSi.

The first adder A1 adds the differential test signal TSS to themultiplied input signal MSi and forms the differential, multiplied inputsignal with test signal component MSiT as shown in FIG. 5 . Preferably,the chopper signal Cs is, as before, a digital signal with two logicalvalues, here exemplarily assigned to 0 and 1.

The differential amplifier DV amplifies the differential, multipliedinput signal with test signal component MSiT to the amplifier outputsignal VO.

The analog-to-digital converter ADC converts the amplifier output signalVO into the input signal DFI of the digital filter DF. This input signalDFI of the digital filter DF is typically a digital signal of samples ofthe amplifier output signal VO from the analog-to-digital convener ADC.

The digital filter OF filters the input signal DFI of the digital filterDF to the output signal DFO of the digital filter DF. In the process,the digital filter DF suppresses any signal components that may bepresent at interference frequencies. Typically, this is a decimationfilter for the conversion artifacts added by the sampling by means ofthe analog-to-digital converter ADC.

The phase compensator PC corrects the resulting phase errors and formsthe phase compensator output signal PCO.

After amplification and digitization, a second multiplier M2 multipliesthe phase compensator output signal PCO with the chopper signal Cs toform the first demodulated signal DM1.

A first low-pass filter LP1 suppresses in the first demodulated signalDM1 the frequencies corresponding to the frequencies in the signalspectrum of the chopper signal Cs. The first low-pass filter LP1suppresses the frequencies corresponding to the frequencies in thesignal spectrum of the orthogonal chopper signal Cs90. The firstlow-pass filter LP1 suppresses the frequencies corresponding to thefrequencies in the signal spectrum of the second chopper signal Cs2. Thefirst low-pass filter LP1 suppresses the mixed frequencies that mayresult from a multiplication of the chopper signal Cs by the orthogonalchopper signal Cs90 and the second chopper signal Cs2. The firstlow-pass filter LP1 suppresses these frequencies except for a DCcomponent in the first demodulated signal DM1. The first low-pass filterLP1 thus forms the first output signal out1. The value of the firstoutput signal out corresponds to the value of the differential inputsignal Si if the sensor of the reference Wheatstone bridge RW is equalto the sensor of the Wheatstone bridge WB.

A third multiplier M3 mixes the first demodulated signal DM1 with theorthogonal chopper signal Cs90 to form the second demodulated signalDM2. A second low-pass filter LP2 suppresses in the second demodulatedsignal DM2 the frequencies corresponding to the frequencies in thesignal spectrum of the chopper signal Cs. The second low-pass filter LP2suppresses the frequencies corresponding to the frequencies in thesignal spectrum of the orthogonal chopper signal Cs90. The secondlow-pass filter LP2 suppresses the frequencies corresponding to thefrequencies in the signal spectrum of the second chopper signal Cs2. Thesecond low-pass filter LP2 suppresses the mixed frequencies that may beproduced by multiplying the chopper signal Cs by the orthogonal choppersignal Cs90 and the second chopper signal Cs2. The second low-passfilter LP2 suppresses these frequencies except for a DC component in thesecond demodulated signal DM2. The second low pass filter LP2 thus formsthe second output signal out2. As before, the value of this secondoutput signal out2 is a measure for the correct function of the inputstage.

A fourth multiplier M4 mixes the first demodulated signal DM1 with thesecond chopper signal Cs2 to form the third demodulated signal DM3. Athird low-pass filter LP3 suppresses the frequencies in the thirddemodulated signal DM3 which correspond to the frequencies in the signalspectrum of the chopper signal Cs. The third low-pass filter LP3suppresses the frequencies corresponding to the frequencies in thesignal spectrum of the orthogonal chopper signal Cs90. The thirdlow-pass filter LP3 suppresses the frequencies corresponding to thefrequencies in the signal spectrum of the second chopper signal Cs2. Thethird low-pass filter LP3 suppresses the mixed frequencies that mayresult from a multiplication of the chopper signal Cs by the orthogonalchopper signal Cs90 and the second chopper signal Cs2. The thirdlow-pass filter LP3 suppresses these frequencies except for a DCcomponent in the third demodulated signal DM3. The third low-pass filterthus forms the third output signal out3. A user or a higher-levelcomputer system or another higher-level system can use the value of thisthird output signal out3 as a measure of the correct operation of theWheatstone bridge WB if the sensor of the reference Wheatstone bridge RWis equal to the sensor of the Wheatstone bridge WB.

In the example of FIG. 5 , a signal generator G1 generates the choppersignal Cs and the orthogonal chopper signal Cs90 and the second choppersignal Cs2. Preferably, the chopper signal Cs is bandwidth-limited ormonofrequency. Preferably, the second chopper signal Cs2 isbandwidth-limited or monofrequency. Preferably, the orthogonal choppersignal Cs90 is bandwidth limited or monofrequency. Preferably, theorthogonal chopper signal Cs90 is different from the chopper signal Cs.Preferably, the second chopper signal Cs2 is different from theorthogonal chopper signal Cs90 and from the chopper signal Cs. The firstlow-pass filter LP1 has a filter property in the form of a first filterfunction F1[] such that: out1=F1[DM1(t)]. The second low-pass filter LP2has a filter property in the form of a second filter function F1[], sothat: out2=F2[DM2(t)]. The third low-pass filter LP3 has a filterproperty in the form of a third filter function F3[], such that:out3=F3[DM3(t)]. Typically, the first low-pass filter LP1 and the secondlow-pass filter LP2 and the third low-pass filter LP3 have a preferablyequal filter property in the form of an equal filter functionF[]=F1[]=F2[]=F3[].

The chopper signal Cs and the second chopper signal Cs2 and theorthogonal chopper signal Cs90 shall each be orthogonal to each otherwith respect to the first filter LP1 and with respect to the secondfilter LP2 and with respect to the third filter LP3. That is, thefollowing shall hold:

i) F1[Cs)t)]=0

ii) F1[Cs90(t)]=0

iii) F1[Cs2(t)]=0

iv) F1[Cs(t)×Cs90(t)]=0

v) F1[Cs(t)×Cs2(t)]=0

vi) F1[Cs90(t)×Cs2(t)]=0

vii) F2[Cs)t)]=0

viii) F2[Cs90(t)]=0

ix) F2[Cs2(t)]=0

x) F2[Cs(t)×Cs90(t)]=0

xi) F2[Cs(t)×Cs2(t)]=0

xii) F2[Cs90(t)×Cs2(t)]=0

vii) F2[Cs)t)]=0

viii) F2[Cs90(t)]=0

ix) F2[Cs2(t)]=0

x) F2[Cs(t)×Cs90(t)]=0

xi) F2[Cs(t)×Cs2(t)]=0

xii) F2[Cs90(t)×Cs2(t)]=0

Here Cs(t) shall represent the time course of the values of the choppersignal Cs and Cs90(t) shall represent the time course of the values ofthe orthogonal chopper signal Cs90 and Cs2(t) shall represent the timecourse of the values of the second chopper signal Cs2.

Furthermore, the first filter function F1[] shall preferably be anessentially linear filter function. That is, the following should holdfor a signal sum of a first example signal X1(t) and a second examplesignal X2(t) and for a real numberα:

A) F1[X1(t)+X2(t)]=B1[X1(t)]+F1[X2(t)]

B) F1[α×X1(t)]=α×F1[X1]

Furthermore, the second filter function F2[] shall preferably be anessentially linear filter function. That is, the following shall holdfor a signal sum of a first example signal X1(t) and a second examplesignal X2(t) and for a real numberα:

C) F2[X1(t)+X2(t)]=B2[X1(t)]+F2[X2(t)]

D) F2[α×X1(t)]=α×F2[X1]

Finally, the third filter function F3[] shall. preferably be anessentially function. That is, the following shall hold for a signal SUMof a first example signal X1(t) and a second example signal X2(t) andfor a real numberα:

E) F3[X1(t)+X2(t)]=F3[X1(t)]+F3[X2(t)]

F) F3[α×X1(t)]=α×F3[X1]

Finally, the first filter function F1[] and the second filter functionF2[] and the third filter function F3[] shall each have a low-passproperty. That is, the following shall hold:

F1[1]=β₁ and F2[1]=β₂ and F3[1]=β₃ withβ₁ as a real non-zero constantandβ₂ as a real non-zero constant andβ₃ as a real non-zero constant.

For example, the chopper signal Cs can be a mono-frequency PWM signalwith the values −1 and 1 and a duty cycle of 50% and a chopper signalfrequency. The orthogonal chopper signal Cs90 can then be, for example,a +/−90° phase-shifted signal with the values −1 and 1 and a duty cycleof 50% with the chopper signal frequency. Alternatively, it can be amono-frequency PWM signal with the values −1 and 1 and a duty cycle of50% and a signal frequency that is, for example, an integer multiple ofthe chopper signal frequency. The chopper signal Cs can also be aband-limited, non-mono-frequency signal, The orthogonal chopper signalCs90 may also be a band-limited, non-mono-frequency signal. Similarly,the second chopper signal Cs2 may be a band-limited, non-mono-frequencysignal. It is only important that the orthogonality conditions i) toxviii) are satisfied. Apart from this, the choice of signals is free.Typically, the chopper signal Cs is periodic and the second choppersignal Cs2 and the orthogonal chopper signal Cs90 are periodic. Ifnecessary, it is useful to provide the first low-pass filter LP1 and thesecond low-pass filter LP2 and the third low-pass filter LP3 each with aholding circuit (English: sample & hold). In this case it makes sense attimes when the orthogonality conditions i) to xviii) are satisfied tosample the output of the first low pass filter LP1 with a first holdingcircuit of these holding circuits. The first holding circuit thenoutputs the first value sampled this way as the value of the firstoutput signal out1 until the next time orthogonality conditions i) toxviii) are satisfied. In addition, at times when orthogonalityconditions i) through xviii) are satisfied, it is useful to sample theoutput of the second low-pass filter LP2 with a second holding circuitof these holding circuits. The second holding circuit then outputs thesecond value sampled this way as the value of the second output signalout2 until the next time orthogonality conditions i) through xviii) aresatisfied. Finally, at times when orthogonality conditions i) throughxviii) are satisfied, it is useful to sample the output of the thirdlow-pass filter LP3 with a third holding circuit of these holdingcircuits. The third holding circuit then outputs the third value sampledin this way as the value of the third output signal out3 until the nexttime orthogonality conditions i) to xviii) are satisfied.

In the example of FIG. 5 , a test signal generator TSG, which can alsobe part of the signal generator G1, generates the test signal TSS fromthe orthogonal chopper signal Cs90. The test signal generator TSG setsthe amplitude according to a preset.

The design of the sensor system can also realize some circuit parts withthe help of a digital circuit or by means of a signal processor systemwith an appropriate programming. This concerns especially the digitalfilter DF, the phase compensation PC, the signal generator G1, the testsignal generator TSG, the second multiplier M2, the third multiplier M3,the fourth multiplier M4, the first low pass filter LP1 and the secondlow pass filter LP2 and the third low pass filter LP3.

Preferably, a comparison means, for example a first comparator incooperation with a second comparator or said signal processor comparesthe value of the second output signal out2 with an expected value rangehounded by a first expected value and a second expected value. If thevalue of the second output signal is between the first expected valueand the second expected value, i.e. within the expected value range, theinput stage comprising the first multiplier M1, the differentialamplifier DV1, the analog-to-digital converter ADC, the digital filterDF, the phase compensator PC and the second multiplier M2 operatescorrectly. Thus, the sensor system or a higher-level computer system orother higher-level device can make a statement about the correctfunction of the input stage in this way.

Preferably, a second comparison means, for example a third comparator incooperation with a fourth comparator or said signal processor comparesthe value of the third output signal out3 with a second expected valuerange bounded by a third expected value and a fourth expected value. Ifthe value of the third output signal out3 is between the third expectedvalue and the fourth expected value, i.e., within the expected valuerange, the Wheatstone bridge WB operates correctly relative to thereference Wheatstone bridge RW. Thus, the sensor system or ahigher-level computer system or other higher-level device can make astatement about the correct function of the Wheatstone bridge WB in thisway.

Thus, the six main operating options are as follows:

The physical parameter (e.g., pressure) affects the reference WheatstoneWheatstone Effect on the Effect on the bridge bridge in the first outputthird output Case design same way. signal out1 signal out3 Comments 1 RW= BW yes Value of out1 out3 ≈ 0 Higher-level returns (correspondssystems can measured toα = 1) use the value value with of out3 to fullbridge detect errors. offset. (corresponds toα = 1) 2 RW ≠ BW yes Valueof out1 Value of out3 Less suitable. RW deviating by returns returnsHigher-level factorα sensitive for measured measured systems can thephysical value with value with use the value parameter(assumption factor(1 + α)/2 factor (1 − α)/2 to detect same bridge offset) with fullwithout errors. bridge offset. bridge offset. 3 RW ≠ BW yes Value ofout1 Value of out3 A RW not sensitive to returns returns ½ measurementthe physical measured measured without parameter value with valuewithout bridge offset (assumption of same factor ½ and bridge offset. ispossible. bridge offset) with full bridge offset. (corresponds toα = 0)4 RW = BW no Value of out1 Value of out3 A returns returns ½ measurementmeasured measured without value with value without bridge offset fullbridge bridge offset. is possible. offset. (corresponds toα = 1) 5 RW ≠BW no Value of out1 Value of out3 A RW deviating by returns returns ½measurement factorα sensitive for measured measured without the physicalvalue with value without bridge offset parameter factor (1 + α)/2 bridgeoffset. is possible (assumption of same with full bridge offset) bridgeoffset. 6 RW ≠ BW no Value of out1 Value of out3 A RW not sensitive toreturns returns ½ measurement the physical measured measured withoutparameter value with value without bridge offset (assumption of samefactor ½ and bridge offset. is possible. bridge offset) with full bridgeoffset. (corresponds toα = 0)

FIG. 6

FIG. 6 corresponds largely to FIG. 5 with the difference that a firstdifferential modulation voltage V_(mod1) from a first voltage source V1and a second voltage source V2, which depends on the test signal TSS,modulates the excitation voltage of the Wheatstone bridge WB. Therespective voltages of the first voltage source V1 and the secondvoltage source V2 preferably depend in mutually opposite ways on thetest signal TSS. Another difference between FIG. 6 and FIG. 5 is that asecond differential modulation voltage V_(mod2) from a third voltagesource V1 b and a fourth voltage source V2 b, which is dependent on thetest signal TSS, modulates the excitation voltage of the referenceWheatstone bridge RW. The respective voltages of the third voltagesource V1 b and the fourth voltage source V2 b preferably depend on thetest signal TSS in mutually opposite ways.

This modulates both the differential input signal Si and thedifferential reference signal Rs proportionally with the test signalTSS. The system of FIG. 6 has the advantage that the tested signal pathincludes the Wheatstone bridge WB and the reference Wheatstone bridgeRW. The disadvantage is the effective reduction of the excitationvoltage and thus the effective reduction of the useful signal swing ofthe input signal Si. The first adder is then omitted. Signal processingis performed in an analogous manner as explained for the precedingfigures.

The differential voltage source that generates the first differentialmodulation voltage in the Wheatstone bridge WB consists, for example, ofa first voltage source V1 and a second voltage source V2. The firstvoltage source V1 is connected between the first resistor R1 and thefirst supply voltage line VDD. The voltage of the first voltage sourceV1 depends on the test signal TSS. The second voltage source V2 isconnected between the third resistor R3 and the first supply voltageline VDD. The voltage of the second voltage source V2 depends on thetest signal TSS. The voltage of the first voltage source V1 depends onthe test signal TSS in the opposite way as the voltage of the secondvoltage source V2. Except for this difference in the sign of thedependence on the test signal TSS, the first voltage source V1 and thesecond voltage source V2 are preferably designed identically.Preferably, they are thermally coupled such that they behavesubstantially the same, Preferably, therefore, the are fabricated on thesame semiconductor substrate.

The differential voltage source that generates the second differentialmodulation voltage V_(mod2) in the reference Wheatstone bridge RWconsists, for example, of a third voltage source V1 b and a fourthvoltage source V2 b. The third voltage source V1 b is connected betweenthe fifth resistor R5 and the first supply voltage line VDD. The voltageof the third voltage source V1 b depends on the test signal TSS. Thefourth voltage source V2 b is connected between the seventh resistor R7and the first supply voltage line VDD. The voltage of the fourth voltagesource V2 b depends on the test signal TSS. Here, the voltage of thethird voltage source V1 b depends on the test signal TSS in the oppositeway as the voltage of the fourth voltage source V2 b. Except for thissign difference of the dependence on the test signal TSS, the thirdvoltage source V1 b and the fourth voltage source V2 b are preferablydesigned identically. Preferably, they are thermally coupled such thatthey behave substantially the same. Preferably, therefore, they are madeof the same semiconductor substrate,

The voltage of the first voltage source V1 depends on the test signalTSS in the same way as the voltage of the third voltage source V1 b. Thevoltage of the second voltage source V2 depends on the test signal TSSin the same way as the voltage of the fourth voltage source V2 b. Thefirst voltage source V1 and the third voltage source V1 b are preferablydesigned in the same way. The second voltage source V2 and the fourthvoltage source V2 b are preferably implemented in the same way.Preferably, all four are thermally coupled such that they behavesubstantially the same except for said sign. Preferably, therefore, theyare made to match on the same semiconductor substrate.

FIG. 7

FIG. 7 largely corresponds to FIG. 6 with the difference that a firstdifferential modulation voltage V_(mod1) dependent on the test signalTSS does not modulate the excitation voltage of the Wheatstone bridgeWB. Here, the first differential modulation voltage V_(mod1) is thedifferential voltage between the output potential of a first voltagesource V1 and the output potential of a second voltage source V2. Therespective voltages of the first voltage source V1 and the secondvoltage source V2 preferably depend on the test signal TSS in mutuallyopposite ways. In contrast to FIG. 6 , the second differentialmodulation voltage dependent on the test signal TSS does not modulatethe excitation voltage of the reference Wheatstone bridge RW. The seconddifferential modulation voltage V_(mod2) is the differential voltagebetween the output potential of a third voltage source V1 b and theoutput potential of a fourth voltage source V2 b. The respectivevoltages of the third voltage source V1 b and the fourth voltage sourceV2 b preferably depend on the test signal TSS in mutually opposite ways.

Instead, the sensor system modulates the value of the first resistor R1and the value of the third resistor R3 in the Wheatstone bridge WB andthe value of the fifth resistor R5 and the value of the seventh resistorR7 in the reference Wheatstone bridge RW.

In FIG. 7 , the sensor system modulates the effective value of the firstresistor R1. For this purpose, the first terminal of the first resistorR1 is connected to the second terminal of a first variable resistor RV1.Furthermore, for this purpose, the first terminal of the first variableresistor RV1 is connected to the first supply voltage line VDD insteadof the first terminal of the first resistor R1. Here, the resistancevalue of the first variable resistor RV1 depends on the value of thetest signal TSS. In the example of FIG. 7 , the test signal TSS switchesa transistor connected in parallel with the resistance value of thefirst variable resistor RV1. For the purpose of this writing, thistransistor and the resistance value connected in parallel to thistransistor together form the first variable resistor RV1. The testsignal TSS inverted by means of a first inverting amplifier INV1controls the transistor of the first variable resistor RV1.

In FIG. 7 , the sensor system modulates the effective value of the thirdresistor R3. For this purpose, the first terminal of the third resistorR3 is connected to the second terminal of a second variable resistorRV2. Furthermore, for this purpose, the first terminal of the secondvariable resistor RV2 is connected to the first supply voltage line VDDinstead of the first terminal of the third resistor R3. The resistancevalue of the second variable resistor RV2 depends here on the value ofthe test signal TSS. In the example of FIG. 7 , the test signal TSSswitches a transistor connected in parallel with the resistance value ofthe second variable resistor RV2. For the purposes of this writing, thistransistor and the resistor value connected in parallel to thistransistor together form the second variable resistor RV2. The testsignal TSS thereby controls the transistor of the second variableresistor RV2.

In FIG. 7 , the sensor system modulates the effective value of the fifthresistor R5. For this purpose, the first terminal of the fifth resistorR5 is connected to the second terminal of a third variable resistor RV3.Furthermore, for this purpose, the first terminal of the third variableresistor RV3 is connected to the first supply voltage line VDD insteadof the first terminal of the fifth resistor R5. The resistance value ofthe third variable resistor RV3 depends here on the value of the testsignal TSS. In the example of FIG. 7 , the test signal TSS switches atransistor connected in parallel with the resistance value of the thirdvariable resistor RV3. For the purposes of this writing, this transistorand the resistance value connected in parallel to this transistortogether form the third variable resistor RV3. The test signal TSSinverted by means of a second inverting amplifier INV2 controls thetransistor of the third variable resistor RV3.

In FIG. 7 , the sensor system modulates the effective value of theseventh resistor R7. For this purpose, the first terminal of the seventhresistor R7 is connected to the second terminal of a fourth variableresistor RV4. Furthermore, for this purpose, the first terminal of thefourth variable resistor RV4 is connected to the first supply voltageline VDD instead of the first terminal of the seventh resistor R7. Theresistance value of the fourth variable resistor RV4 depends here on thevalue of the test signal TSS. In the example of FIG. 7 , the test signalTSS switches a transistor connected in parallel with the resistancevalue of the fourth variable resistor RV4. For the purposes of thiswriting, this transistor and the resistor value connected in parallel tothis transistor together form the fourth variable resistor RV4. The testsignal TSS thereby controls the transistor of the fourth variableresistor RV4.

By this exemplary construction, the test signal TSS modulates both thedifferential input signal Si and the differential reference signal Rsproportionally. The system of FIG. 7 has the advantage that the testedsignal path includes the Wheatstone bridge WB and the referenceWheatstone bridge RW. The disadvantage is the effective reduction of theexcitation voltage and thus the effective reduction of the stroke. Thefirst adder is then omitted. Signal processing is performed in ananalogous manner as explained for the preceding figures.

Preferably, the resistance values of the first variable resistor RV1 andthe third variable resistor RV3 depend on the test signal TSS in thesame way.

Preferably, the resistance values of the second variable resistor RV2and the fourth variable resistor RV4 depend on the test signal TSS inthe same way.

Preferably, the resistance values of the first variable resistor RV1 andthe second variable resistor RV2 depend on the test signal TSS in aninverse but otherwise identical manner.

Preferably, the resistance values of the third variable resistor RV3 andthe fourth variable resistor RV4 depend on the test signal TSS in aninverse but otherwise identical manner.

Preferably, the first variable resistor RV1 is designed to be equal(English: matching) to the second variable resistor RV2.

Preferably, the third variable resistor RV3 is designed to be equal(English: matching) to the fourth variable resistor RV4.

Preferably, the first variable resistor RV1 is designed to be equal(English: matching) to the third variable resistor RV3.

Preferably, the second variable resistor RV2 is designed to be equal(English: matching) to the fourth variable resistor RV4.

LIST OF REFERENCE SYMBOLS

-   -   A1 first adder;    -   ADC analog-to-digital converter;    -   Cs chopper signal;    -   Cs2 second chopper signal;    -   Cs90 orthogonal chopper signal;    -   DF digital filter;    -   DFI input signal of the digital filter (DF);    -   DFO output signal of the digital filter (DF);    -   DM1 first demodulated signal;    -   DM2 second demodulated signal;    -   DM3 third demodulated signal;    -   DS Dicke switch;    -   DV differential amplifier;    -   G1 signal generator;    -   GND second supply voltage line;    -   INV1 first inverting amplifier or inverter;    -   INV2 second inverting amplifier or inverter;    -   LP1 first low pass filter;    -   LP2 second inverting amplifier or inverter;    -   LP3 third low pass filter;    -   M1 first multiplier;    -   M2 second multiplier;    -   M3 third multiplier;    -   M4 fourth multiplier;    -   MSi multiplied input signal;    -   MSiT differential, multiplied input signal with test signal        component;    -   out1 first output signal;    -   out2 second output signal;    -   out3 third output signal;    -   PC phase compensator;    -   PCO phase compensator output signal;    -   R1 first resistor;    -   R2 second resistor;    -   R3 third resistor;    -   R4 fourth resistor;    -   R5 fifth resistor;    -   R6 sixth resistor;    -   R7 seventh resistor;    -   R8 eighth resistor;    -   Rin negative reference signal;    -   Rip positive reference    -   Rs reference signal;    -   RV1 first variable resistor;    -   RV2 second variable resistor;    -   RV3 third variable resistor;    -   RV4 fourth variable resistor;    -   RW reference Wheatstone bridge;    -   Si differential input signal;    -   Sin negative input signal;    -   Sip positive input signal;    -   SiT differential input signal with test signal component;    -   T time;    -   TSG test signal generator;    -   TSS test signal;    -   WB Wheatstone bridge consisting of first resistor (R1), second        resistor (R2), third resistor (R3) and fourth resistor (R4);    -   V1 first voltage source;    -   V1 b third voltage source;    -   V2 second voltage source;    -   V2 b fourth voltage source;    -   V_(mod1) first differential modulation voltage;    -   V_(mod2) second differential modulation voltage;    -   VO amplifier output signal;    -   VDD first supply voltage line;

LIST OF CITED DOCUMENTS Patent Literature

-   -   EP 2 524 389 B1,    -   EP 2 524 390 B1,    -   EP 2 524 198 B1,    -   EP 2 523 896 B1,    -   EP 2 523 895 B1,

Non-Patent Literature

-   -   Script “Introduction to Differential Geometry” by Christopher R.        Nerz

Links

-   -   https://de.wikipedia.org/wiki/Lp-Raum#Der_Hilbertraum_L2    -   https://www.math.uni-tuebingen.de/de/forschung/gadr/lehre/sose2015/diffgeo.pdf

What is claimed is:
 1. A method for monitoring a sensor system inoperation: wherein the sensor system comprises a sensor element (WB)providing an input signal (Si) with a time course (Si(t)) of an inputsignal value of the input signal (Si); wherein the sensor systemcomprises a signal path; wherein the signal path comprises, at a firstlocation on the signal path, an amplifier (DV) having an input and anoutput; wherein the signal path starts with the input signal (Si) fromthe sensor element (WB); and wherein the signal path ends with a firstoutput signal (out1) of the sensor system and wherein a value of thefirst output signal (out1) or a value of a signal derived therefromrepresents a measured value; comprising: a) first mixing of the inputsignal (Si) in the signal path with a chopper signal (Cs) at a secondlocation in the signal path; wherein the second location is in thesignal path between the sensor element (WB) and the input of theamplifier (DV); and where the chopper signal (Cs) is band-limited ormono-frequency; b) second mixing of the input signal (Si) at a thirdlocation in the signal path with the chopper signal (Cs) to form a firstdemodulated signal (DM1); wherein a third position in the signal path isbetween the output of the amplifier (DV) at the first position in thesignal path and the first output signal (out1) of the sensor system atthe end of the signal path; c) first filtering of the first demodulatedsignal (DM1) or a signal derived therefrom at a fourth position in thesignal path between the third position in the signal path on the onehand and the first output signal (out1) of the sensor system at the endof the signal path on the other hand; wherein said first filtering isperformed by applying a first filter function F1[] to the firstdemodulated signal (DM1) or the signal derived therefrom; wherein thefirst filter function F1[] describes a relationship between a timecourse (DM1(t)) of the first demodulated signal (DM1) or of the signalderived therefrom on the one hand and a time course of the signalimmediately after the first filtering; and wherein the first outputsignal (out1) depends on or is a result of said first demodulated signal(DM1) immediately after said first filtering; characterized by theadditional steps and further comprising: d) adding an orthogonal choppersignal (Cs90) to the input signal (Si) in the signal path at a fifthlocation in the signal path between the sensor element (WB) and theamplifier (DV); wherein the chopper signal (Cs) has a time course(Cs(t)) of the chopper signal (Cs); wherein the orthogonal choppersignal (Cs90) has a time course (Cs90(t)) of the orthogonal choppersignal (Cs90); and wherein the time course (Cs90(t)) of the orthogonalchopper signal (Cs90) with respect to said first filter function F1[]substantially has, except for noise and similar signal errors, aproperty F1[Cs90(t)×Cs(t)]=0 at least at times; e) third mixing of thefirst demodulated signal (DM1) or a signal derived therefrom with theorthogonal chopper signal (Cs90) or a signal derived therefrom andgenerating a second demodulated signal (DM2); f) second filtering of thesecond demodulated signal (DM2) or a signal derived therefrom by meansof a second filter function F2[] to a second output signal (out2);wherein the second filter function F2[] is chosen such that followingessentially hold:F2[Cs(t)]=0,F2[Cs90(t)]=0,F2[Cs(t)×Cs90(t)]=0, andF2[1]=β₂ with β₂ as real or complex value and the first filter functionF1[] is chosen such that following essentially hold:F1[Cs(t)]=0,F1[Cs90(t)]=0,F1[Cs(t)×Cs90(t)]=0, andF1[1]=β₁ with β₁ as real or complex value; g) first comparing of a valueof the second output signal (out2) or a value of a signal derivedtherefrom with an expected value interval and concluding on an error ifthe value of the second output signal (out2) or the signal derivedtherefrom is outside the expected value interval.
 2. The methodaccording to claim 1 further comprising: (h) providing a referenceelement (RW) that provides a reference signal (Rs); i) processing thereference signal (Rs) in a reference signal path; wherein the referencesignal path is designed equal to the signal path for processing theinput signal (Si); wherein the reference signal path starts with thereference signal (Rs); wherein the reference signal path ends with thesecond output signal (out2); wherein the reference signal path at abeginning of the reference signal path at the reference signal (Rs) isdifferent from the signal path at a beginning of the signal path at theinput signal (Si); wherein the reference signal path comprises, at afirst location of the reference signal path, the amplifier (DV) havingthe input and the output; wherein the amplifier (DV) is thus part of thereference signal path at the first location in the reference signal pathand part of the signal path at the first location in the signal path;wherein the reference signal path has a changeover switch (DS) with afirst input and a second input at a sixth position in the referencesignal path between the reference element (RW) and the input of theamplifier (DV), the sixth position in the reference signal pathcorresponding to a sixth position in the signal path, so that thechangeover switch (DS) is also arranged at the a same time at the sixthposition in the signal path between the sensor element (WB) and theinput of the amplifier (DV), and thus the changeover switch (DS) is acommon changeover switch (DS); wherein the common changeover switch (DS)selects an active input in dependence on a second chopper signal (Cs2)between its first input and its second input; wherein the signal pathcomprises the first input of the changeover switch (DS); wherein thereference signal path comprises the second input of the changeoverswitch (DS); wherein the signal path does not include the second inputof the changeover switch (DS); wherein the reference signal path doesnot include the first input of the changeover switch (DS); wherein thecommon changeover switch (DS) selects its active input respectively independence on the second chopper signal (Cs2) and switches through arespective current value at this respectively selected active input ofthe common changeover switch (DS) to an output of the common changeoverswitch (DS); wherein the reference signal path and the signal path areidentical in a section from the output of the common changeover switch(DS) at the sixth position in the reference signal path and at the sixthposition in the signal path on the one hand and the input of theamplifier (DV) at the first position in the reference signal path andsignal path on the other hand; and wherein the first filtering with thefirst filter function F1[] is only part of the signal path and not partof the reference signal path; j) fourth mixing the first demodulatedsignal (DM1) or a signal derived therefrom with the second choppersignal (Cs2) to form a third demodulated signal (DM3); k) thirdfiltering of the third demodulated signal (DM3) or a signal derivedtherefrom by means of a third filter function F3[] to a third outputsignal (out3); wherein the first filter function F1[] is chosen suchthat following essentially hold:F1[Cs(t)]=0,F1[Cs2(t)]=0,F1[Cs90(t)]=0,F1[Cs(t)×Cs2(t)]=0,F1[Cs(t)×Cs90(t)]=0F1[Cs2(t)×Cs90(t)]=0,F1[Cs(t)×Cs2(t)×Cs90(t)]=0, andF1[1]=β₁ with β₁ as real or complex value holds; wherein the secondfilter function F2[] is selected such that following essentially hold:F2[Cs(t)]=0,F2[Cs2(t)]=0,F2[Cs90(t)]=0,F2[Cs(t)×Cs2(t)]=0,F2[Cs(t)×Cs90(t)]=0,F2[Cs2(t)×Cs90(t)]=0,F2[Cs(t)×Cs2(t)×Cs90(t)]=0, and F2[1]=β₂ with β₂ as real or complexvalue; and wherein the third filter function F3[] is chosen such thatfollowing essentially hold:F3[Cs(t)]=0,F3[Cs2(t)]=0,F3[Cs90(t)]=0,F3[Cs(t)×Cs2(t)]=0,F3[Cs(t)×Cs90(t)]=0,F3[Cs2(t)×Cs90(t)]=0,F3[Cs(t)×Cs2(t)×Cs90(t)]=0, and F3[1]=β₃ with β₃ as real or complexvalue; and l) second comparing of a value of the third output signal(out3) or a value of a signal derived therefrom with a third expectedvalue interval and concluding an error if the value of the third outputsignal (out3) or the value of the signal derived therefrom is outsidethe third expected value interval.
 3. A method for monitoring a sensorsystem in operation: wherein the sensor system comprises a sensorelement (WB) providing an input signal (Si) having an input signal valuein dependence on a test signal (TSS); wherein the sensor systemcomprises a signal path; wherein the signal path comprises, at a firstlocation in the signal path, an amplifier (DV) having an input and anoutput; wherein the signal path starts with the input signal (Si) fromthe sensor element (WB); wherein the signal path ends with a firstoutput signal (out1); and wherein a value of the output signal (out1)represents a measured value; comprising: a) first mixing of the inputsignal (Si) with a chopper signal (Cs) at a second point in the signalpath; wherein he second location is in the signal path between thesensor element (WB) and the input of the amplifier (DV); and wherewherein the chopper signal (Cs) is band-limited or mono-frequency; b)second mixing of the input signal at a third location in the signal pathwith the chopper signal (Cs) to form a first demodulated signal (DM1);wherein the third location is located in the signal path between theoutput of the amplifier (DV) on the one hand and the first output signal(out1) of the sensor system on the other hand; c) first filtering of thefirst demodulated signal (DM1) or a signal derived therefrom at a fourthposition in the signal path between the third position in the signalpath on the one hand and the output signal (out1) at the end of thesignal path on the other hand; wherein said first filtering is performedby applying a first filter function F1[] to the first demodulated signal(DM1) or the signal derived therefrom; wherein the first filter functionF1[] describes a relationship between a time course (DM1(t)) of thefirst demodulated signal (DM1) or the signal derived therefrom on theone hand and a time course of the input signal (Si) on the other hand,this relationship between the time course (DM1(t)) of the firstdemodulated signal (DM1) or of the signal derived therefrom and the timecourse of the input signal (Si) being a result of filtering the firstdemodulated signal (DM1) with the first filter function F1[]; andwherein the first output signal (out1) depends on this first demodulatedsignal (DM1) or is the result of this first filtering; the methodcomprising: characterized by the steps, d) generating the test signal(TSS) in dependence on an orthogonal chopper signal (Cs90); wherein thechopper signal (Cs) has a time course (Cs(t)) of the chopper signal(Cs); wherein the orthogonal chopper signal (Cs90) has a time course(Cs90(t)) of the orthogonal chopper signal (Cs90); and wherein the timecourse (Cs90(t)) of the orthogonal chopper signal (Cs90) with respect tosaid first filter function F1[] substantially has, except for noise andsimilar signal errors, a property F1[Cs90(t)×Cs(t)]=0 at least at times;e) third mixing of the first demodulated signal (DM1) or the signalderived therefrom with the orthogonal chopper signal (Cs90) or a signalderived therefrom and generating a second demodulated signal (DM2); f)second filtering of the second demodulated signal (DM2) or a signalderived therefrom by means of a second filter function F2[] to a secondoutput signal (out2); wherein the second filter function F2[] is chosensuch that following essentially hold:F2[Cs(t)]=0,F2[Cs90(t)]=0,F2[Cs90(t)×Cs90(t)]=0,F2[Cs(t)×Cs90(t)]=0,F2[Cs(t)×Cs(t)]=0, andF2[1]=β₂ with β₂ as real or complex value; and wherein the first filterfunction F1[] is chosen such that following essentially hold:F1[Cs(t)]=0,F1[Cs90(t)]=0,F1[Cs(t)×Cs90(t)]=0, andF1[1]=β₁ with β₁ as a real or complex value; and g) first comparing thevalue of the second output signal (out2) or the value of a signalderived therefrom with an expected value interval and concluding on anerror if the value of the second output signal (out2) or the signalderived therefrom is outside the expected value interval.
 4. The methodaccording to claim 3, further comprising: (h) providing a referenceelement (RW) that provides a reference signal (Rs); i) processing thereference signal (Rs) in a reference signal path; wherein the referencesignal path is designed equal to the signal path for processing theinput signal (Si); wherein the reference signal path starts with thereference signal (Rs); wherein the reference signal path ends with thesecond output signal (out2); wherein the reference signal path at abeginning of the reference signal path at the reference signal (Rs) isdifferent from the signal path at the beginning of the signal path atthe input signal Si; wherein the reference signal path comprises, at afirst location of the reference signal path, the amplifier (DV) havingthe input and the output; wherein thus the amplifier (DV) is part of thereference signal path at the first location in the reference signal pathand part of the signal path at the first location in the signal path;wherein at a sixth position in the reference signal path between thereference element (RW) and the input of the amplifier (DV), whichcorresponds to a sixth position in the signal path between the sensorelement (WB) and the input of the amplifier (DV), a changeover switch(DS), which is common to the signal path and the reference signal pathand has a first input and a second input, is inserted; wherein thecommon changeover switch (DS) selects its first input or its secondinput as a respective active input in dependence on a second choppersignal (Cs2); wherein the signal path comprises the first input of thechangeover switch (DS); wherein the reference signal path comprises thesecond input of the changeover switch (DS); wherein the signal path doesnot include the second input of the changeover switch (DS); wherein thereference signal path does not include the first input of the changeoverswitch (DS); wherein the common changeover switch (DS) selects itsactive input selected in dependence on the second chopper signal (Cs2)and switching through an the output of the common changeover switch (DS)a value at this active input of the common changeover switch (DS)corresponding to the input signal (Si) or to the reference signal (Rs);wherein the reference signal path and the signal path are identical in asection from the output of the common switch (DS) on the one hand; andthe input of the amplifier (DV) on the other hand; and wherein the firstfiltering with the first filter function F1[] is not part of thereference signal path; j) fourth mixing of the first demodulated signal(DM1) or a signal derived therefrom with the second chopper signal (Cs2)to form a third demodulated signal (DM3); k) third filtering of thethird demodulated signal (DM3) or a signal derived therefrom by means ofa third filter function F3[] to a third output signal (out3); whereinthe first filter function F1[] is chosen such that following essentiallyhold:F1[Cs(t)]=0,F1[Cs2(t)]=0,F1[Cs90(t)]=0,F1[Cs(t)×Cs2(t)]=0,F1[Cs(t)×Cs90(t)]=0F1[Cs2(t)×Cs90(t)]=0,F1[Cs(t)×Cs2(t)×Cs90(t)]=0, andF1[1]=β₁ with β₁ as real or complex value holds; and wherein the secondfilter function F2[] is selected such that following essentially hold:F2[Cs(t)]=0,F2[Cs2(t)]=0,F2[Cs90(t)]=0,F2[Cs(t)×Cs2(t)]=0,F2[Cs(t)×Cs90(t)]=0,F2[Cs2(t)×Cs90(t)]=0,F2[Cs(t)×Cs2(t)×Cs90(t)]=0, and F2[1]=β₂ with β₂ as real or complexvalue; and wherein the third filter function F3[] is chosen such thatfollowing essentially hold:F3[Cs(t)]=0,F3[Cs2(t)]=0,F3[Cs90(t)]=0,F3[Cs(t)×Cs2(t)]=0,F3[Cs(t)×Cs90(t)]=0,F3[Cs2(t)×Cs90(t)]=0,F3[Cs(t)×Cs2(t)×Cs90(t)]=0, and F3[1]=β₃ with β₃ as real or complexvalue; and l) second comparing of a value of the third output signal(out3) or a value of a signal derived therefrom with a third expectedvalue interval and concluding an error if the value of the third outputsignal (out3) or the value of the signal derived therefrom is outsidethe third expected value interval.
 5. A pressure sensor for use in themethod according to claim 4; wherein the pressure sensor comprises asensor element (WB) in the form of a Wheatstone bridge with fourpiezoresistive resistors (R1, R2, R3, R4); wherein the pressure sensorcomprises a reference element (RW) in the form of a reference Wheatstonebridge with four piezoresistive reference resistors (R5, R6, R7, R8);wherein the reference resistors (R5, R6, R7, R8) of the referenceWheatstone bridge (RW) are arranged in a same way as the resistors (R1,R2, R3, R4) of the Wheatstone bridge; wherein the pressure sensor isdisposed on a monolithic crystal; wherein the pressure sensor comprisesa cavity closed on one side by a membrane; wherein the resistors (R1,R2, R3, R4) of the Wheatstone bridge (WB) are arranged at leastpartially on the membrane; wherein the reference resistors (R5, R6, R7,R8) of the reference Wheatstone bridge (RW) are not arranged on themembrane; wherein a first resistor (R1) of the Wheatstone bridge (WB) issimilar to a fifth resistor (R5) of the reference Wheatstone bridge (RW)in that it is constructed in a same manner; wherein a second resistor(R2) of the Wheatstone bridge (WB) is similar to a sixth resistor (R6)of the reference Wheatstone bridge (RW) in that it is constructed in asame manner; wherein a third resistor (R3) of the Wheatstone bridge (WB)is similar to a seventh resistor (R7) of the reference Wheatstone bridge(RW) in that it is constructed in a same manner; wherein a fourthresistor (R4) of the Wheatstone bridge (WB) is similar to an eighthresistor (R8) of the reference Wheatstone bridge (RW) in that it isconstructed in a same manner; and wherein the reference WheatstoneBridge (RW) is used as a reference noise source for subsequent signalprocessing.
 6. A pressure sensor for use in the method according toclaim 4; wherein the pressure sensor comprises a sensor element (WB) inthe form of a Wheatstone bridge with four piezoresistive resistors (R1,R2, R3, R4); wherein the pressure sensor comprises a reference element(RW) in the form of a reference Wheatstone bridge with fourpiezoresistive reference resistors (R5, R6, R7, R8); wherein thereference resistors (R5, R6, R7, R8) of the reference Wheatstone bridge(RW) are arranged in the a same way as the resistors (R1, R2, R3, R4) ofthe Wheatstone bridge; wherein the pressure sensor is disposed on amonolithic crystal; wherein the pressure sensor comprises a first cavityclosed on a first side by a first membrane; wherein the first cavity hasa cavity surface of the first cavity opposite the first side of thefirst cavity; wherein the pressure sensor comprises a reference cavityclosed on a second side by a second membrane; wherein the referencecavity has a cavity surface of the reference cavity opposite the secondside of the reference cavity; wherein the resistors (R1, R2, R3, R4) ofthe Wheatstone bridge (WB) are arranged at least partially on the firstmembrane; wherein the reference resistors (R5, R6, R7, R8) of thereference Wheatstone bridge (RW) are arranged at least partially on thesecond membrane; wherein a first resistor (R1) of the Wheatstone bridge(WB) is similar to a fifth resistor (R5) of the reference Wheatstonebridge (RW) in that it is constructed in a same manner; wherein a secondresistor (R2) of the Wheatstone bridge (WB) is similar to a sixthresistor (R6) of the reference Wheatstone bridge (RW) in that it isconstructed in a same manner; wherein a third resistor (R3) of theWheatstone bridge (WB) is similar to a seventh resistor (R7) of thereference Wheatstone bridge (RW) in that it is constructed in a samemanner; wherein a fourth resistor (R4) of the Wheatstone bridge (WB) issimilar to an eighth resistor (R8) of the reference Wheatstone bridge(RW) in that it is constructed in a same manner; wherein the referenceWheatstone bridge (RW) is used as a reference noise source forsubsequent signal processing; and wherein the first membrane isdifferent in design from the second membrane and/or wherein the firstcavity differs in its design from the reference cavity and/or whereinthe cavity surface of the first cavity opposite the first side of thefirst cavity is configured differently from the cavity surface of thereference cavity opposite the second side of the reference cavity;and/or wherein the first cavity and the reference cavity are each filledwith a fluid, wherein the fluid in the first cavity is different fromthe fluid in the reference cavity, or the fluid in the first cavity isin a different state than the fluid in the reference cavity, whereinvacuum is considered to be the fluid.
 7. A sensor for use in the methodaccording to claim 4: wherein the sensor comprises a first resistor (R1)having a first terminal and a second terminal; wherein the sensorcomprises a second resistor (R2) having a first terminal and a secondterminal; wherein the sensor comprises a third resistor (R3) having afirst terminal and a second terminal; wherein the sensor comprises afourth resistor (R4) having a first terminal and a second terminal;wherein the sensor comprises a first voltage source (V1) having a firstterminal and a second terminal; wherein the sensor comprises a secondvoltage source (V2) having a first terminal and a second terminal;wherein the first terminal of the first voltage source (V1) is connectedto a first supply voltage line (VDD); wherein the second terminal of thefirst voltage source (V1) is connected to the first terminal of thefirst resistor (R1); wherein the second terminal of the first resistor(R1) is connected to the first terminal of the second resistor (R2);wherein the second terminal of the second resistor is connected to asecond supply voltage line (GND); wherein the first terminal of thesecond voltage source (V2) is connected to the first supply voltage line(VDD); wherein the second terminal of the second voltage source (V2) isconnected to the first terminal of the third resistor (R3); wherein thesecond terminal of the third resistor (R3) is connected to the firstterminal of the fourth resistor (R4); wherein the second terminal of thefourth resistor (R4) is connected to the second supply voltage line(GND); wherein a first voltage of the first voltage source (V1) dependson a test signal (TSS); and wherein a second voltage of the secondvoltage source (V2) depends on the test signal (TSS) in an inversemanner to the first voltage of the first voltage source (V1).
 8. Asensor, for use in the method according to claim 4: wherein the sensorcomprises a first resistor (R1) having a first terminal and a secondterminal; wherein the sensor comprises a second resistor (R2) having afirst terminal and a second terminal; wherein the sensor comprises athird resistor (R3) having a first terminal and a second terminal;wherein the sensor comprises a fourth resistor (R4) having a firstterminal and a second terminal; wherein the sensor comprises a firstvariable resistor (RV1) having a first terminal and a second terminal;wherein the sensor comprises a second variable resistor (RV2) having afirst terminal and a second terminal; wherein the first terminal of thefirst variable resistor (RV1) is connected to a first supply voltageline (VDD); wherein the second terminal of the first variable resistor(RV1) is connected to the first terminal of the first resistor (R1);wherein the second terminal of the first resistor (R1) is connected tothe first terminal of the second resistor (R2); wherein the secondterminal of the second resistor is connected to a second supply voltageline (GND); wherein the first terminal of the second variable resistor(RV2) is connected to the first supply voltage line (VDD); wherein thesecond terminal of the second variable resistor (RV2) is connected tothe first terminal of the third resistor (R3); wherein the secondterminal of the third resistor (R3) is connected to the first terminalof the fourth resistor (R4); wherein the second terminal of the fourthresistor (R4) is connected to the second supply voltage line (GND);wherein a resistance value of the first variable resistor (RV1) dependson a test signal (TSS); and wherein a resistance value of the secondvariable resistor (RV2) depends on the test signal (TSS) in an inversemanner to the resistance value of the first variable resistor (RV1). 9.The sensor according to claim 8, wherein the sensor is a pressuresensor.
 10. The sensor according to claim 7, wherein the sensor is apressure sensor.
 11. A pressure sensor for use in the method accordingto claim 2: wherein the pressure sensor comprises a sensor element (WB)in the form of a Wheatstone bridge with four piezoresistive resistors(R1, R2, R3, R4); wherein the pressure sensor comprises a referenceelement (RW) in the form of a reference Wheatstone bridge with fourpiezoresistive reference resistors (R5, R6, R7, R8); wherein thereference resistors (R5, R6, R7, R8) of the reference Wheatstone bridge(RW) are arranged in a same way as the resistors (R1, R2, R3, R4) of theWheatstone bridge (WB); wherein the pressure sensor is disposed on amonolithic crystal; wherein the pressure sensor comprises a cavityclosed on one side by a membrane; wherein the resistors (R1, R2, R3, R4)of the Wheatstone bridge (WB) are arranged at least partially on themembrane; wherein the reference resistors (R5, R6, R7, R8) of thereference Wheatstone bridge (RW) are not arranged on the membrane;wherein a first resistor (R1) of the Wheatstone bridge (WB) is similarto a fifth resistor (R5) of the reference Wheatstone bridge (RW) in thatit is constructed in a same manner; wherein a second resistor (R2) ofthe Wheatstone bridge (WB) is similar to a sixth resistor (R6) of thereference Wheatstone bridge (RW) in that it is constructed in a samemanner; wherein a third resistor (R3) of the Wheatstone bridge (WB) issimilar to a seventh resistor (R7) of the reference Wheatstone bridge(RW) in that it is constructed in a same manner; wherein a fourthresistor (R4) of the Wheatstone bridge (WB) is similar to an eighthresistor (R8) of the reference Wheatstone bridge (RW) in that it isconstructed in a same manner; and wherein the reference WheatstoneBridge (RW) is used as a reference noise source for subsequent signalprocessing.
 12. A pressure sensor for use in the method according toclaim 2: wherein the pressure sensor comprises a sensor element (WB) inthe form of a Wheatstone bridge with four piezoresistive resistors (R1,R2, R3, R4); wherein the pressure sensor comprises a reference element(RW) in the form of a reference Wheatstone bridge with fourpiezoresistive reference resistors (R5, R6, R7, R8); wherein thereference resistors (R5, R6, R7, R8) of the reference Wheatstone bridge(RW) are arranged in the a same way as the resistors (R1, R2, R3, R4) ofthe Wheatstone bridge (WB); wherein the pressure sensor is disposed on amonolithic crystal; wherein the pressure sensor comprises a first cavityclosed on a first side by a first membrane; wherein the first cavity hasa cavity surface of the first cavity opposite the first side of thefirst cavity; wherein the pressure sensor comprises a reference cavityclosed on a second side by a second membrane; wherein the referencecavity has a cavity surface of the reference cavity opposite the secondside of the reference cavity; wherein the resistors (R1, R2, R3, R4) ofthe Wheatstone bridge (WB) are arranged at least partially on the firstmembrane; wherein the reference resistors (R5, R6, R7, R8) of thereference Wheatstone bridge (RW) are arranged at least partially on thesecond membrane; wherein a first resistor (R1) of the Wheatstone bridge(WB) is similar to a fifth resistor (R5) of the reference Wheatstonebridge (RW) in that it is constructed in a same manner; wherein a secondresistor (R2) of the Wheatstone bridge (WB) is similar to a sixthresistor (R6) of the reference Wheatstone bridge (RW) in that it isconstructed in a same manner; wherein a third resistor (R3) of theWheatstone bridge (WB) is similar to a seventh resistor (R7) of thereference Wheatstone bridge (RW) in that it is constructed in a samemanner; wherein a fourth resistor (R4) of the Wheatstone bridge (WB) issimilar to an eighth resistor (R8) of the reference Wheatstone bridge(RW) in that it is constructed in a same manner; wherein the referenceWheatstone bridge (RW) is used as a reference noise source forsubsequent signal processing; and wherein the first membrane isdifferent in design from the second membrane; and/or wherein the firstcavity differs in its design from the reference cavity; and/or whereinthe cavity surface of the first cavity opposite the first side of thefirst cavity is configured differently from the cavity surface of thereference cavity opposite the second side of the reference cavity;and/or wherein the first cavity and the reference cavity are each filledwith a fluid, wherein the fluid in the first cavity is different fromthe fluid in the reference cavity, or the fluid in the first cavity isin a different state than the fluid in the reference cavity, whereinvacuum is considered to be the fluid.
 13. A sensor for use in the methodaccording to claim 3: wherein the sensor comprises a first resistor (R1)having a first terminal and a second terminal; wherein the sensorcomprises a second resistor (R2) having a first terminal and a secondterminal; wherein the sensor comprises a third resistor (R3) having afirst terminal and a second terminal; wherein the sensor comprises afourth resistor (R4) having a first terminal and a second terminal;wherein the sensor comprises a first voltage source (V1) having a firstterminal and a second terminal; wherein the sensor comprises a secondvoltage source (V2) having a first terminal and a second terminal;wherein the first terminal of the first voltage source (V1) is connectedto a first supply voltage line (VDD); wherein the second terminal of thefirst voltage source (V1) is connected to the first terminal of thefirst resistor (R1); wherein the second terminal of the first resistor(R1) is connected to the first terminal of the second resistor (R2);wherein the second terminal of the second resistor is connected to asecond supply voltage line (GND); wherein the first terminal of thesecond voltage source (V2) is connected to the first supply voltage line(VDD); wherein the second terminal of the second voltage source (V2) isconnected to the first terminal of the third resistor (R3); wherein thesecond terminal of the third resistor (R3) is connected to the firstterminal of the fourth resistor (R4); wherein the second terminal of thefourth resistor (R4) is connected to the second supply voltage line(GND); wherein a first voltage of the first voltage source (V1) dependson a test signal (TSS); and wherein a second voltage of the secondvoltage source (V2) depends on the test signal (TSS) in an inversemanner to the first voltage of the first voltage source (V1).
 14. Asensor-for use in the method according to claim 3: wherein the sensorcomprises a first resistor (R1) having a first terminal and a secondterminal; wherein the sensor comprises a second resistor (R2) having afirst terminal and a second terminal; wherein the sensor comprises athird resistor (R3) having a first terminal and a second terminal;wherein the sensor comprises a fourth resistor (R4) having a firstterminal and a second terminal; wherein the sensor comprises a firstvariable resistor (RV1) having a first terminal and a second terminal;wherein the sensor comprises a second variable resistor (RV2) having afirst terminal and a second terminal; wherein the first terminal of thefirst variable resistor (RV1) is connected to a first supply voltageline (VDD); wherein the second terminal of the first variable resistor(RV1) is connected to the first terminal of the first resistor (R1);wherein the second terminal of the first resistor (R1) is connected tothe first terminal of the second resistor (R2); wherein the secondterminal of the second resistor is connected to a second supply voltageline (GND); wherein the first terminal of the second variable resistor(RV2) is connected to the first supply voltage line (VDD); wherein thesecond terminal of the second variable resistor (RV2) is connected tothe first terminal of the third resistor (R3); wherein the secondterminal of the third resistor (R3) is connected to the first terminalof the fourth resistor (R4); wherein the second terminal of the fourthresistor (R4) is connected to the second supply voltage line (GND);wherein a resistance value of the first variable resistor (RV1) dependson a test signal (TSS); and wherein a resistance value of the secondvariable resistor (RV2) depends on the test signal (TSS) in an inversemanner to the resistance value of the first variable resistor (RV1).